unicode/utf8: add AppendRune

AppendRune appends the UTF-8 encoding of a rune to a []byte.
It is a generally more user friendly than EncodeRune.

    EncodeASCIIRune-4     2.35ns ± 2%
    EncodeJapaneseRune-4  4.60ns ± 2%
    AppendASCIIRune-4     0.30ns ± 3%
    AppendJapaneseRune-4  4.70ns ± 2%

The ASCII case is written to be inlineable.

Fixes #47609

Change-Id: If4f71eedffd2bd4ef0d7f960cb55b41c637eec54
Reviewed-on: https://go-review.googlesource.com/c/go/+/345571


Trust: Joe Tsai <joetsai@digital-static.net>
Reviewed-by: Rob Pike <r@golang.org>
Run-TryBot: Rob Pike <r@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>