I copied the code from various dependencies in go-moremath
into a single 'internal/stats' package. That package is at the top
level of the repo because I expect to pull much of benchcmp
into an importable package.

For golang/go#14304.

Change-Id: Ie114839b2901f5060c202feb3ffc768bf43ce5da
Reviewed-on: https://go-review.googlesource.com/35503
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Quentin Smith <quentin@golang.org>