In golang.org/cl/42813, a test was added in the bytes package to check
if a Buffer method was being inlined, using 'go tool nm'.

Now that we have a compiler test that verifies that certain funcs are
inlineable, merge it there. Knowing whether the funcs are inlineable is
also more reliable than whether or not their symbol appears in the
binary, too. For example, under some circumstances, inlineable funcs
can't be inlined, such as if closures are used.

While at it, add a few more bytes.Buffer methods that are currently
inlined and should clearly stay that way.

Updates #21851.

Change-Id: I62066e32ef5542d37908bd64f90bda51276da4de
Reviewed-on: https://go-review.googlesource.com/65658
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Marvin Stenger <marvin.stenger94@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>