use GAT to redesign trait Stream

[Sherlock-Holo]

for now Stream can't write something like
https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=b45f2bc81394150cdae15d8a1cbcdecd

which want to return a reference type,

however, with GAT help
https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=b63d58ebe8d9ffa70225d5fc23fb629d

we can return the reference type

consider Stream still not in the std lib, maybe we still have chance to redesign it

[taiki-e]

Since wg-async is investigating/discussing the new design of stream trait, I transfer this there.

[Berrysoft]

Stream is actually in the std, which names std::async_iter::AsyncIterator. It's a nightly feature.

[Sherlock-Holo]

AsyncIterator is not stable, that means still have a chance to redesign it, like the try trait