Improve error message when neither std nor alloc is enabled

[dtolnay]

Some tweaks to make the following produce a more compact error.

[dependencies]
serde_json = { version = "1.0", default-features = false }

After:

error: expected item, found `"serde_json requires that either `std` (default) or `alloc` feature is enabled"`
 --> src/features_check/error.rs:1:1
  |
1 | "serde_json requires that either `std` (default) or `alloc` feature is enabled"
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected item

Before:

error: expected item, found `"serde_json requires that either `std` (default) or `alloc` feature is enabled"`
 --> src/features_check/error.rs:1:1
  |
1 | "serde_json requires that either `std` (default) or `alloc` feature is enabled"
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected item

error[E0407]: method `visit_string` is not a member of trait `Visitor`
  --> src/value/de.rs:60:13
   |
60 | /             fn visit_string<E>(self, value: String) -> Result<Value, E> {
61 | |                 Ok(Value::String(value))
62 | |             }
   | |_____________^ not a member of trait `Visitor`

error[E0407]: method `visit_string` is not a member of trait `Visitor`
    --> src/value/de.rs:1340:5
     |
1340 | /     fn visit_string<E>(self, s: String) -> Result<Self::Value, E>
1341 | |     where
1342 | |         E: de::Error,
1343 | |     {
...    |
1350 | |         }
1351 | |     }
     | |_____^ not a member of trait `Visitor`

error[E0277]: the trait bound `alloc::string::String: serde::ser::Serialize` is not satisfied
   --> src/map.rs:301:22
    |
301 |             tri!(map.serialize_entry(k, v));
    |                      ^^^^^^^^^^^^^^^ the trait `serde::ser::Serialize` is not implemented for `alloc::string::String`

error[E0277]: the trait bound `alloc::string::String: serde::de::Deserialize<'de>` is not satisfied
   --> src/map.rs:337:61
    |
337 |                 while let Some((key, value)) = tri!(visitor.next_entry()) {
    |                                                             ^^^^^^^^^^ the trait `serde::de::Deserialize<'de>` is not implemented for `alloc::string::String`

error[E0599]: no method named `visit_string` found for struct `value::de::<impl serde::de::Deserialize<'de> for value::Value>::deserialize::ValueVisitor` in the current scope
  --> src/value/de.rs:56:22
   |
22 |         struct ValueVisitor;
   |         -------------------- method `visit_string` not found for this
...
56 |                 self.visit_string(String::from(value))
   |                      ^^^^^^^^^^^^ method not found in `value::de::<impl serde::de::Deserialize<'de> for value::Value>::deserialize::ValueVisitor`

error[E0277]: the trait bound `alloc::string::String: serde::de::Deserialize<'de>` is not satisfied
   --> src/value/de.rs:115:69
    |
115 |                         while let Some((key, value)) = tri!(visitor.next_entry()) {
    |                                                                     ^^^^^^^^^^ the trait `serde::de::Deserialize<'de>` is not implemented for `alloc::string::String`

error[E0599]: no method named `visit_string` found for type parameter `V` in the current scope
   --> src/value/de.rs:211:41
    |
211 |             Value::String(v) => visitor.visit_string(v),
    |                                         ^^^^^^^^^^^^ help: there is an associated function with a similar name: `visit_str`

error[E0599]: no method named `visit_string` found for type parameter `V` in the current scope
   --> src/value/de.rs:341:41
    |
341 |             Value::String(v) => visitor.visit_string(v),
    |                                         ^^^^^^^^^^^^ help: there is an associated function with a similar name: `visit_str`

error[E0599]: no method named `visit_string` found for type parameter `V` in the current scope
   --> src/value/de.rs:358:41
    |
358 |             Value::String(v) => visitor.visit_string(v),
    |                                         ^^^^^^^^^^^^ help: there is an associated function with a similar name: `visit_str`

error[E0599]: no method named `visit_string` found for type parameter `V` in the current scope
    --> src/value/de.rs:1228:52
     |
1228 |                 (Err(_), Cow::Owned(s)) => visitor.visit_string(s),
     |                                                    ^^^^^^^^^^^^ help: there is an associated function with a similar name: `visit_str`
...
1244 |     deserialize_integer_key!(deserialize_i8 => visit_i8);
     |     ----------------------------------------------------- in this macro invocation
     |
     = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0599]: no method named `visit_string` found for type parameter `V` in the current scope
    --> src/value/de.rs:1228:52
     |
1228 |                 (Err(_), Cow::Owned(s)) => visitor.visit_string(s),
     |                                                    ^^^^^^^^^^^^ help: there is an associated function with a similar name: `visit_str`
...
1245 |     deserialize_integer_key!(deserialize_i16 => visit_i16);
     |     ------------------------------------------------------- in this macro invocation
     |
     = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0599]: no method named `visit_string` found for type parameter `V` in the current scope
    --> src/value/de.rs:1228:52
     |
1228 |                 (Err(_), Cow::Owned(s)) => visitor.visit_string(s),
     |                                                    ^^^^^^^^^^^^ help: there is an associated function with a similar name: `visit_str`
...
1246 |     deserialize_integer_key!(deserialize_i32 => visit_i32);
     |     ------------------------------------------------------- in this macro invocation
     |
     = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0599]: no method named `visit_string` found for type parameter `V` in the current scope
    --> src/value/de.rs:1228:52
     |
1228 |                 (Err(_), Cow::Owned(s)) => visitor.visit_string(s),
     |                                                    ^^^^^^^^^^^^ help: there is an associated function with a similar name: `visit_str`
...
1247 |     deserialize_integer_key!(deserialize_i64 => visit_i64);
     |     ------------------------------------------------------- in this macro invocation
     |
     = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0599]: no method named `visit_string` found for type parameter `V` in the current scope
    --> src/value/de.rs:1228:52
     |
1228 |                 (Err(_), Cow::Owned(s)) => visitor.visit_string(s),
     |                                                    ^^^^^^^^^^^^ help: there is an associated function with a similar name: `visit_str`
...
1248 |     deserialize_integer_key!(deserialize_u8 => visit_u8);
     |     ----------------------------------------------------- in this macro invocation
     |
     = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0599]: no method named `visit_string` found for type parameter `V` in the current scope
    --> src/value/de.rs:1228:52
     |
1228 |                 (Err(_), Cow::Owned(s)) => visitor.visit_string(s),
     |                                                    ^^^^^^^^^^^^ help: there is an associated function with a similar name: `visit_str`
...
1249 |     deserialize_integer_key!(deserialize_u16 => visit_u16);
     |     ------------------------------------------------------- in this macro invocation
     |
     = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0599]: no method named `visit_string` found for type parameter `V` in the current scope
    --> src/value/de.rs:1228:52
     |
1228 |                 (Err(_), Cow::Owned(s)) => visitor.visit_string(s),
     |                                                    ^^^^^^^^^^^^ help: there is an associated function with a similar name: `visit_str`
...
1250 |     deserialize_integer_key!(deserialize_u32 => visit_u32);
     |     ------------------------------------------------------- in this macro invocation
     |
     = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0599]: no method named `visit_string` found for type parameter `V` in the current scope
    --> src/value/de.rs:1228:52
     |
1228 |                 (Err(_), Cow::Owned(s)) => visitor.visit_string(s),
     |                                                    ^^^^^^^^^^^^ help: there is an associated function with a similar name: `visit_str`
...
1251 |     deserialize_integer_key!(deserialize_u64 => visit_u64);
     |     ------------------------------------------------------- in this macro invocation
     |
     = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0599]: no method named `visit_string` found for type parameter `V` in the current scope
    --> src/value/de.rs:1228:52
     |
1228 |                 (Err(_), Cow::Owned(s)) => visitor.visit_string(s),
     |                                                    ^^^^^^^^^^^^ help: there is an associated function with a similar name: `visit_str`
...
1254 |         deserialize_integer_key!(deserialize_i128 => visit_i128);
     |         --------------------------------------------------------- in this macro invocation
     |
     = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0599]: no method named `visit_string` found for type parameter `V` in the current scope
    --> src/value/de.rs:1228:52
     |
1228 |                 (Err(_), Cow::Owned(s)) => visitor.visit_string(s),
     |                                                    ^^^^^^^^^^^^ help: there is an associated function with a similar name: `visit_str`
...
1255 |         deserialize_integer_key!(deserialize_u128 => visit_u128);
     |         --------------------------------------------------------- in this macro invocation
     |
     = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0599]: no method named `visit_string` found for type parameter `V` in the current scope
    --> src/value/de.rs:1395:43
     |
1395 |             Cow::Owned(string) => visitor.visit_string(string),
     |                                           ^^^^^^^^^^^^ help: there is an associated function with a similar name: `visit_str`

error[E0277]: the trait bound `alloc::string::String: serde::ser::Serialize` is not satisfied
  --> src/value/ser.rs:27:30
   |
27 |                     tri!(map.serialize_entry(k, v));
   |                              ^^^^^^^^^^^^^^^ the trait `serde::ser::Serialize` is not implemented for `alloc::string::String`

error: aborting due to 22 previous errors