- avoid copy in range ytab
- add fast path to prefixof

Change-Id: I88aa9d91a0abe80d253f7c3bca950b4613297499
Reviewed-on: https://go-review.googlesource.com/7628
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Rob Pike <r@golang.org>