math/bits: faster OnesCount

math/bits: faster OnesCount

Using some additional suggestions per "Hacker's Delight".
Added documentation and extra tests.

Measured on 1.7 GHz Intel Core i7, running macOS 10.12.3.

benchmark                  old ns/op     new ns/op     delta
BenchmarkOnesCount-4       7.34          5.38          -26.70%
BenchmarkOnesCount8-4      2.03          1.98          -2.46%
BenchmarkOnesCount16-4     2.56          2.50          -2.34%
BenchmarkOnesCount32-4     2.98          2.39          -19.80%
BenchmarkOnesCount64-4     4.22          2.96          -29.86%

Change-Id: I566b0ef766e55cf5776b1662b6016024ebe5d878
Reviewed-on: https://go-review.googlesource.com/37223
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>