bytes: clean-up of buffer.go

bytes: clean-up of buffer.go

Clean-up changes in no particular order:
- use uint8 instead of int for readOp
- remove duplicated code in ReadFrom()
- introduce (*Buffer).empty()
- remove naked returns

Change-Id: Ie6e673c20c398f980f8be0448969a36ad4778804
Reviewed-on: https://go-review.googlesource.com/42816
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>