encoding/gob: rename encBuffer.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: I5b509eb7f0118d5f2d3c6e352ff2849cd5a3071e
Reviewed-on: https://go-review.googlesource.com/c/go/+/176110


Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>