tokio: introduce I/O utility helpers

[carllerche]

The existing I/O utility helpers were deleted in #1164. New ones should be implemented.

The utility helpers will be implemented on AsyncReadExt and AsyncWriteExt traits and will exist in tokio::io.

Some helpers to add include, but are not limited to:

AsyncReadExt

• [x] read
• [x] read_to_end
• [x] read_to_string
• [x] read_exact
• [x] read_until
• [x] copy

AsyncWriteExt

• [x] write
• [x] write_all
• [x] flush

[bIgBV]

I'm working on porting over read_until but it depends on the std::io::BufRead trait and the now deprecated try_nb! macro from tokio-io. I'm not sure how the BufRead bound will work with the new syntax.

One option is to copy over the functionality of read_until from BufRead into this future. This would mean adding a dependency on memchr though, as it looks like the standard library ships with its own version of the same.

[bIgBV]

Looks like read_to_end also depended on behavior from the Read trait and will have to be re-thought as well.

[taiki-e]

I think it is good to define the AsyncBufRead trait and experiment with it, like futures-io.

EDIT: Opened #1301

[mokomull]

Does AsyncRead::split() belong on this list as well? It seems to have been replaced with a TCP-only implementation, but I was using it with UnixStream.

I am currently using futures 0.3's AsyncReadExt::split instead, via @Nemo157's shim from #1297, and it works beautifully.

[carllerche]

@mokomull added a line item to #1256.