Fix clippy::needless_lifetimes warning (#113)

```
warning: the following explicit lifetimes could be elided: 'a
  --> async-stream/src/yielder.rs:90:6
   |
90 | impl<'a, T> Drop for Enter<'a, T> {
   |      ^^                    ^^
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_lifetimes
   = note: `#[warn(clippy::needless_lifetimes)]` on by default
help: elide the lifetimes
   |
90 - impl<'a, T> Drop for Enter<'a, T> {
90 + impl<T> Drop for Enter<'_, T> {
   |

warning: the following explicit lifetimes could be elided: 'a
   --> async-stream/tests/stream.rs:164:19
    |
164 |         fn stream<'a>(&'a self) -> impl Stream<Item = &str> + 'a {
    |                   ^^   ^^                              ^      ^^
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_lifetimes
    = note: `#[warn(clippy::needless_lifetimes)]` on by default
help: elide the lifetimes
    |
164 -         fn stream<'a>(&'a self) -> impl Stream<Item = &str> + 'a {
164 +         fn stream(&self) -> impl Stream<Item = &str> + '_ {
    |
```

async-stream/src/yielder.rs

@@ -87,7 +87,7 @@ impl<T> Receiver<T> {
 
 // ===== impl Enter =====
 
-impl<'a, T> Drop for Enter<'a, T> {
+impl<T> Drop for Enter<'_, T> {
     fn drop(&mut self) {
         STORE.with(|cell| cell.set(self.prev));
     }

async-stream/tests/stream.rs

@@ -161,7 +161,7 @@ async fn borrow_self() {
     struct Data(String);
 
     impl Data {
-        fn stream<'a>(&'a self) -> impl Stream<Item = &str> + 'a {
+        fn stream(&self) -> impl Stream<Item = &str> + '_ {
             stream! {
                 yield &self.0[..];
             }