This is needed for some of the more complex primality tests
(to filter out exact squares), and while the code is simple the
boundary conditions are not obvious, so it seems worth having
in the library.

Change-Id: Ica994a6b6c1e412a6f6d9c3cf823f9b653c6bcbd
Reviewed-on: https://go-review.googlesource.com/30706
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>