math/bits: faster OnesCount using table lookups for sizes 8,16,32

math/bits: faster OnesCount using table lookups for sizes 8,16,32

For uint64, the existing algorithm is faster.

benchmark                  old ns/op     new ns/op     delta
BenchmarkOnesCount8-8      1.95          0.97          -50.26%
BenchmarkOnesCount16-8     2.54          1.39          -45.28%
BenchmarkOnesCount32-8     2.61          1.96          -24.90%

Measured on 2.3 GHz Intel Core i7 running macOS 10.12.3.

Change-Id: I6cc42882fef3d24694720464039161e339a9ae99
Reviewed-on: https://go-review.googlesource.com/37580
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>