Remove hyper::body::Chunk, replace with bytes::Bytes

[davidbarsky]

Resolves #1931

This PR is a take at removing hyper::body::Chunk and replacing it with bytes::Bytes. I think that this PR should probably rely on bytes 0.5, but that hasn't been published yet. Additionally, there are still mentions of "chunk" in non-public facing code; that should probably be scrubbed.

cc: @abonander

[weihanglo]

Some lines in h1::io benchmark need to remove crate::Chunk.

hyper/src/proto/h1/io.rs

Lines 926 to 929 in db1ba07

	let mut write_buf = WriteBuf::<crate::Chunk>::new();
	write_buf.set_strategy(WriteStrategy::Flatten);
	b.iter(|| {
	let chunk = crate::Chunk::from(s);

[lnicola]

Nb. bytes 0.5 is now published.

[davidbarsky]

@lnicola Thanks for the heads up!

[seanmonstar]

We could move forward with this, thought there's some merge conflicts. I have a thing or two left that are distracting me, but I can fix this up after if you don't have time.

[seanmonstar]

Fixed up here: #2048

[davidbarsky]

@seanmonstar Thanks for doing that! Sorry about that delay; I was pulled into an operation issue.