Fix clippy::used_underscore_items warning

```
error: used underscore-prefixed item
   --> tests/stream.rs:144:11
    |
144 |     yield foo::_Foo(0);
    |           ^^^^^^^^^^^^
    |
note: item is defined here
   --> tests/stream.rs:139:5
    |
139 |     pub struct _Foo(pub i32);
    |     ^^^^^^^^^^^^^^^
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#used_underscore_items
    = note: `-D clippy::used-underscore-items` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(clippy::used_underscore_items)]`

error: used underscore-prefixed item
   --> tests/stream.rs:145:11
    |
145 |     yield foo::_Foo(1);
    |           ^^^^^^^^^^^^
    |
note: item is defined here
   --> tests/stream.rs:139:5
    |
139 |     pub struct _Foo(pub i32);
    |     ^^^^^^^^^^^^^^^
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#used_underscore_items

error: used underscore-prefixed item
   --> tests/stream.rs:158:11
    |
158 |     yield foo::_Foo(0);
    |           ^^^^^^^^^^^^
    |
note: item is defined here
   --> tests/stream.rs:139:5
    |
139 |     pub struct _Foo(pub i32);
    |     ^^^^^^^^^^^^^^^
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#used_underscore_items

error: used underscore-prefixed item
   --> tests/stream.rs:159:11
    |
159 |     yield foo::_Foo(1);
    |           ^^^^^^^^^^^^
    |
note: item is defined here
   --> tests/stream.rs:139:5
    |
139 |     pub struct _Foo(pub i32);
    |     ^^^^^^^^^^^^^^^
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#used_underscore_items
```

tests/stream.rs

@@ -136,33 +136,33 @@ async fn stream3() {
 }
 
 mod foo {
-    pub struct _Foo(pub i32);
+    pub struct Foo(pub i32);
 }
 
-#[stream(boxed, item = foo::_Foo)]
+#[stream(boxed, item = foo::Foo)]
 pub async fn stream5() {
-    yield foo::_Foo(0);
-    yield foo::_Foo(1);
+    yield foo::Foo(0);
+    yield foo::Foo(1);
 }
 
 #[stream(item = i32, boxed)]
 pub async fn stream6() {
     #[for_await]
-    for foo::_Foo(i) in stream5() {
+    for foo::Foo(i) in stream5() {
         yield i * i;
     }
 }
 
-#[stream(boxed_local, item = foo::_Foo)]
+#[stream(boxed_local, item = foo::Foo)]
 pub async fn stream7() {
-    yield foo::_Foo(0);
-    yield foo::_Foo(1);
+    yield foo::Foo(0);
+    yield foo::Foo(1);
 }
 
 #[stream(item = i32, boxed_local)]
 pub async fn stream8() {
     #[for_await]
-    for foo::_Foo(i) in stream5() {
+    for foo::Foo(i) in stream5() {
         yield i * i;
     }
 }