Add rpc::all function

[Thomasdezeeuw]

Goals:

• Minimal number of wakeups of the caller (ideally only 1 when all calls have return or errored).
• Minimal amount of allocations.

API idea

/// Run multple RPCs concurrently.
fn all<I, Res>(rpc: I) -> Res
where
    I: ExactSizeIterator<RPC<T>>,
    Res: FromIterator<Result<T, SendError>>,
{
    // ...
}

Specially design task::Waker implementation that only wakes if all responses are ready, or if one errored (?).
Waker called:

• If message not yet send -> wake to send message.
• If message is already send -> don't wake, mark the message as ready to receive.