Add normal `read()` / `peek()` async functions to `AsyncBufReadExt`

[sdroege]

This would fill the internal buffer until the given amount is reached and then provide access to it. read() could return a value that implements Deref<Target=[u8]> and on Drop consumes the read data, peek() would only provide direct access to an &[u8].

This seems useful to have to allow copy-less operation on AsyncBufRead impls. If they implement AsyncRead in addition, a copy would always be necessary.

Inspired partially by https://boats.gitlab.io/blog/post/io-uring/

[Nemo157]

What is a buffered reader to do if the requested amount exceeds the internal buffer size?

[sdroege]

Return less, just like AsyncRead::read() can return less

[Nemo157]

So this could purely be an AsyncBufReadExt sugar around poll_fill_buf + consume, not something implemented for each AsyncBufRead?

peek seems like it would just be the missing AsyncBufReadExt::fill_buf (which IIRC might have lifetime issues).

[sdroege]

Yes that is exactly what I meant, sorry for not being more clear :)

[taiki-e]

I think this is addressed by #2225.