strconv: add QuoteToGraphic and friends

strconv: add QuoteToGraphic and friends

This version of quoting allows runes in category Zs, such as the
ideographic space characters, to be passed through unquoted.

Still to do (maybe): A way to access this from Printf.

Updates #11511.

Change-Id: I3bae84b1aa0bc1b885318d3f67c5f451099a2a5a
Reviewed-on: https://go-review.googlesource.com/14184
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>