fmt: rename buffer.WriteByte to writeByte

Renaming the method makes clear, both to readers and to vet,
that this method is not the implementation of io.ByteWriter.

Working toward making the tree vet-safe instead of having
so many exceptions in cmd/vet/all/whitelist.

For #31916.

Change-Id: I79da062ca6469b62a6b9e284c6cf2413c7425249
Reviewed-on: https://go-review.googlesource.com/c/go/+/176109


Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>