Add separate `AsyncReceiver` type instead of implementing `Future` directly on the `Receiver`

[faern]

This PR adds a separate type, AsyncReceiver, that implements Future, instead of implementing it directly on the Receiver type. Now the Receiver implements IntoFuture instead.

This change removes the possible panics in many recv* methods, and it simplifies some code a bit.

This is a breaking change. So it is scheduled for the 0.2 release.