Name of Buf::to_bytes does not follow self conventions

[mzabaluev]

An inherent method with the signature of Buf::to_bytes would not pass clippy:

methods called to_* usually take self by reference; consider choosing a less ambiguous name
help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#wrong_self_convention

In the next breaking release, consider renaming the method or maybe replace it with #129 (though less deceptively named than copy_to_bytes).

[seanmonstar]

Hm, it is true it doesn't follow that convention (though I don't think that's too big a deal in itself). I don't think of any convenient other names that explain what it does. Possibly something like take_bytes or take_into_bytes
...

[taiki-e]

Fixed by #439