all: use strings.Builder where appropriate

all: use strings.Builder where appropriate

Change-Id: I164d350ca480640996055dedf38d962921c474a6
Reviewed-on: https://go-review.googlesource.com/c/go/+/435975
Run-TryBot: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Run-TryBot: hopehook <hopehook@golangcn.org>
TryBot-Result: Gopher Robot <gobot@golang.org>