io: read_to_end and write_all, or more generally Async{Read,Write}Ext

[pchickey]

When I'm using tokio the two methods I use most frequently, and are missing from wstd, are:

https://docs.rs/tokio/latest/tokio/io/trait.AsyncReadExt.html#method.read_to_end https://docs.rs/tokio/latest/tokio/io/trait.AsyncWriteExt.html#method.write_all

We need at very least to provide implementations of those in wstd.

Maybe we should provide a complete copy of the AsyncReadExt and AsyncWriteExt traits, since there are many other bits in there which people use.

@yoshuawuyts says:

compat with tokio would be great. If you want we can probably build in some
compat methods similar to https://docs.rs/async-compat/latest/async_compat/.
Maybe even behind a flag?

[pchickey]

Yosh and I discussed, and concluded to just expose read_to_end and write_all as builtin methods on the AsyncRead/AsyncWrite traits. The rest of the Ext stuff is YAGNI - when someone complains we could consider adding it, I guess, but its not critical.