This is the simple half of https://golang.org/cl/53560043/ with
a new benchmark. pongad is in the C+A files already.

benchmark                         old ns/op     new ns/op     delta
BenchmarkReaderWriteToOptimal     2054          825           -59.83%

Update #6373

LGTM=iant, gri
R=golang-codereviews, iant, gri
CC=golang-codereviews
https://golang.org/cl/69220046