encoding/json: rely on utf8.AppendRune

encoding/json: rely on utf8.AppendRune

Change-Id: I50e5609ff9c5f2b216b93cec7fb5214d196cae90
Reviewed-on: https://go-review.googlesource.com/c/go/+/412537
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Run-TryBot: Joseph Tsai <joetsai@digital-static.net>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Joseph Tsai <joetsai@digital-static.net>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>