Motivations for this change:
* It allows these functions to be used outside of Reader/Writer.
* It allows these functions to be more easily unit tested.

Change-Id: Iebe2b70bdb8744371c9ffa87c24316cbbf025b59
Reviewed-on: https://go-review.googlesource.com/15113Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Joe Tsai <joetsai@digital-static.net>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>