strconv: rely on utf8.AppendRune

strconv: rely on utf8.AppendRune

This is both simpler and more performant.

Quote             268ns ± 5%   258ns ± 4%  -3.70%  (p=0.014 n=10+10)
QuoteRune        28.9ns ± 3%  28.4ns ± 4%    ~     (p=0.113 n=9+10)
AppendQuote       165ns ± 3%   165ns ± 3%    ~     (p=0.661 n=9+10)
AppendQuoteRune  8.05ns ± 5%  7.75ns ± 7%    ~     (p=0.065 n=10+9)

Change-Id: Ib0ee332e970d4986026c05e5e0e368f41eff7977
Reviewed-on: https://go-review.googlesource.com/c/go/+/412338
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Joseph Tsai <joetsai@digital-static.net>
Reviewed-by: Robert Griesemer <gri@google.com>