Automatically convert to external errors

A pattern documented by thiserror is

```
pub enum MyError {
  ...

  #[error(transparent)]
  Other(#[from] anyhow::Error), // source and Display delegate to anyhow::Error
}
```

It'd be nice for this to work with ensure! but right now, that macro returns
an eyre error.

With this PR, the macro additionally runs an .into(). In the case that
the return type is an eyre error, obviously .into() will do nothing and
be compiled away. In the case that there is a from method, the wrapping
will occur. This enables eyre to be used for ergonomic 'implementation
detail error' in a thiserror using system which has contractual errors.

src/macros.rs

@@ -51,13 +51,13 @@
 #[macro_export]
 macro_rules! bail {
     ($msg:literal $(,)?) => {
-        return $crate::private::Err($crate::eyre!($msg));
+        return $crate::private::Err($crate::eyre!($msg).into());
     };
     ($err:expr $(,)?) => {
-        return $crate::private::Err($crate::eyre!($err));
+        return $crate::private::Err($crate::eyre!($err).into());
     };
     ($fmt:expr, $($arg:tt)*) => {
-        return $crate::private::Err($crate::eyre!($fmt, $($arg)*));
+        return $crate::private::Err($crate::eyre!($fmt, $($arg)*).into());
     };
 }
 
@@ -109,17 +109,17 @@ macro_rules! bail {
 macro_rules! ensure {
     ($cond:expr, $msg:literal $(,)?) => {
         if !$cond {
-            return $crate::private::Err($crate::eyre!($msg));
+            return $crate::private::Err($crate::eyre!($msg).into());
         }
     };
     ($cond:expr, $err:expr $(,)?) => {
         if !$cond {
-            return $crate::private::Err($crate::eyre!($err));
+            return $crate::private::Err($crate::eyre!($err).into());
         }
     };
     ($cond:expr, $fmt:expr, $($arg:tt)*) => {
         if !$cond {
-            return $crate::private::Err($crate::eyre!($fmt, $($arg)*));
+            return $crate::private::Err($crate::eyre!($fmt, $($arg)*).into());
         }
     };
 }

tests/test_macros.rs

@@ -39,6 +39,22 @@ fn test_ensure() {
         Ok(())
     };
     assert!(f().is_err());
+
+    let f = || -> Result<(), SomeWrappingErr> {
+        ensure!(false, "this will fail");
+        Ok(())
+    };
+    assert!(f().is_err());
+}
+
+struct SomeWrappingErr {
+    err: eyre::Error,
+}
+
+impl From<eyre::Error> for SomeWrappingErr {
+    fn from(err: eyre::Error) -> Self {
+        SomeWrappingErr { err }
+    }
 }
 
 #[test]