Implement Stream::size_hint method on stream utilities

[taiki-e]

Most of async-std's stream utilities were written before Stream::size_hint was released and do not implement Stream::size_hint method. This means that async-std's stream utilities may be a bit inefficient than other crates such as futures, tokio-stream, futures-lite, which provide similar utilities.

related: rust-lang/futures-rs#1853, rust-lang/rust#81797

[mich2000]

Hey guys I wanted to know if on the size_hint method in the stream trait on the crate, if I should give it a default implementation. Something like below, a default size_hint is implemented in the stream module in the standard library.

fn size_hint(&self) -> (usize, Option) {
(0, None)
};

[taiki-e]

IIRC, this crate's Stream trait is for documentation, so I think either one is fine.

[mich2000]

Okay, I'll directly work on the stream size_hint implementation.

[mich2000]

PR: #967