all: use strings.Builder instead of bytes.Buffer

all: use strings.Builder instead of bytes.Buffer

Change-Id: I6d7213d29ade591b8366d5640833bd5a20474165
GitHub-Last-Rev: 94195832873fe6764d45ffdd6efa1ca49e57a743
GitHub-Pull-Request: golang/go#54834
Reviewed-on: https://go-review.googlesource.com/c/go/+/427814
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: hopehook <hopehook@golangcn.org>
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Heschi Kreinick <heschi@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>