Use a manual Future impl for methods

[yoshuawuyts]

I'm wrapping Receiver::recv and Sender::send in structs. Because they're async fn instead of manual Future impls they require boxing when stored. It'd be nice if we could expose two new types:

• RecvFuture
• SendFuture

These would not require boxing, which means they can be inlined in implementations and would increase overall perf when wrapping async-channel. Given the simplicity of send and recv I suspect the implementation to be rather straight forward.

Thanks!

[yoshuawuyts]

Ran into this again today; still stand by that this is a change we should implement.

[fogti]

This would probably require similarly exposed Future impls in event-listener.

[yoshuawuyts]

Filed a PR here: #33