Fixes tokio-rs#4337

Rust 1.57 stabilized the `Iterator::map_while` API. This PR adds the
same functionality to the `StreamExt` trait, to keep parity.

This way we can take leverage the Option optimization where for non-null
types we don't need any extra space to represent the `None`-ness.
Whether this applies will depend on the specific `St` type, but it's
very common.

`MapWhile` doesn't need to remember whether it's done. As per the contract of the `Stream` trait:

> Once a stream has finished (returned Ready(None) from poll_next), calling its poll_next method again may panic, block forever, or cause other kinds of problems; the Stream trait places no requirements on the effects of such a call.

This allows us to save the space that may be needed by the boolean,
but also lets us simplify the implementation. People can use `fuse`
if they so need.

Co-authored-by: Taiki Endo <te316e89@gmail.com>