Implement IntoFuture for tuples of IntoFutures
Implement IntoFuture for tuples of IntoFutures

This way, instead of having to call the correct `join$n` function, one can just
call `(a, b, c, ...).into_future()` (or just pass the tuple to a function
expecting an `IntoFuture`.

Note: There should probably also be an implementation on `(F,)` for completeness
but that's not very useful outside of macros.

Idolf/futures-rs,alexcrichton/futures-rs