Add `stream::Zip` trait about futures-concurrency

[yoshuawuyts]

For the Zip variant: we may want to skip directly ahead to #22 in terms of implementation, creating an output slot of Container[MaybUninit<T>] where we write into, and then mem::swap on each iteration.

Metadata would need to be stored in an accompanying PollState slot. The main things to keep an eye will be to:

• ensure to implement the PinnedDrop trait to drop memory in the case of a cancellation
• switch between PollState::Pending and PollState::Ready while data is actively being yielded, and only switch to PollState::Consume at the end.

The semantics of Zip are that if at any point one of the streams yields Poll::Ready(None), all other streams are cancelled. We should create a case where one stream yields more times than the other streams so that we can exercise miri's checkers during CI.

[yoshuawuyts]

Filed a concern in rust-lang/rust#80094 since its existence requires us to author:

let mut s = Zip::zip([a, b, c]);

instead of:

let mut s = [a, b, c].zip();