io: depend on bytes 1.0

[carllerche]

Before releasing Tokio 1.0, the bytes dependency needs to be updated to bytes 1.0.

[jonhoo]

Related bytes milestone for 1.0: https://github.com/tokio-rs/bytes/milestone/3

[taiki-e]

Also, bytes 0.6 milestone still has some open issues: https://github.com/tokio-rs/bytes/milestone/2

[carllerche]

@taiki-e good call. We should triage them.

[taiki-e]

In addition, bytes 0.5 milestone also has some open issues: https://github.com/tokio-rs/bytes/milestone/1
(I didn't notice it because the milestone was closed.)