Fix clippy::needless_lifetimes warning

```
error: the following explicit lifetimes could be elided: 'a
    --> src/lib.rs:1197:6
     |
1197 | impl<'a, T> EventListenerFuture for SendInner<'a, T> {
     |      ^^                                       ^^
     |
     = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_lifetimes
     = note: `-D clippy::needless-lifetimes` implied by `-D warnings`
     = help: to override `-D warnings` add `#[allow(clippy::needless_lifetimes)]`
help: elide the lifetimes
     |
1197 - impl<'a, T> EventListenerFuture for SendInner<'a, T> {
1197 + impl<T> EventListenerFuture for SendInner<'_, T> {
     |

error: the following explicit lifetimes could be elided: 'a
    --> src/lib.rs:1253:6
     |
1253 | impl<'a, T> EventListenerFuture for RecvInner<'a, T> {
     |      ^^                                       ^^
     |
     = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_lifetimes
help: elide the lifetimes
     |
1253 - impl<'a, T> EventListenerFuture for RecvInner<'a, T> {
1253 + impl<T> EventListenerFuture for RecvInner<'_, T> {
     |
```

src/lib.rs

@@ -1194,7 +1194,7 @@ pin_project! {
     }
 }
 
-impl<'a, T> EventListenerFuture for SendInner<'a, T> {
+impl<T> EventListenerFuture for SendInner<'_, T> {
     type Output = Result<(), SendError<T>>;
 
     /// Run this future with the given `Strategy`.
@@ -1250,7 +1250,7 @@ pin_project! {
     }
 }
 
-impl<'a, T> EventListenerFuture for RecvInner<'a, T> {
+impl<T> EventListenerFuture for RecvInner<'_, T> {
     type Output = Result<T, RecvError>;
 
     /// Run this future with the given `Strategy`.