io: export StringWriter

And start using it elsewhere in the standard library, removing the
copies in the process.

While at it, rewrite the io.WriteString godoc to be more clear, since it
can now make reference to the defined interface.

Fixes #27946.

Change-Id: Id5ba223c09c19e5fb49815bd3b1bd3254fc786f3
Reviewed-on: https://go-review.googlesource.com/c/139457


Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>