encoding: use strings.Builder

encoding: use strings.Builder

Change-Id: Ib07699df8ea41fd8d1bca8ad050859fac24623de
Reviewed-on: https://go-review.googlesource.com/c/go/+/428258
Reviewed-by: Robert Griesemer <gri@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>