This is like a write-only subset of bytes.Buffer with an
allocation-free String method.

Fixes #18990.

Change-Id: Icdf7240f4309a52924dc3af04a39ecd737a210f4
Reviewed-on: https://go-review.googlesource.com/74931
Run-TryBot: Caleb Spare <cespare@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>