Improve BytesMut::split suggestion

[paolobarbolini]

Incorporates the improvement from #698 into the must_use suggestion of BytesMut::split.

[Darksonn]

You know, this makes me realize that advance could be simplified to call clear instead of set_len(0).

[paolobarbolini]

You know, this makes me realize that advance could be simplified to call clear instead of set_len(0).

Yeah I had skipped clear because it can't avoid the branch inside truncate, but I guess if that was a real problem it could be solved within clear itself.