math/bits: faster Reverse8/16 functions using table lookups

math/bits: faster Reverse8/16 functions using table lookups

Measured on 2.3 GHz Intel Core i7, running macOS 10.12.3:

benchmark                old ns/op     new ns/op     delta
BenchmarkReverse8-8      1.70          0.99          -41.76%
BenchmarkReverse16-8     2.24          1.32          -41.07%

Fixes #19279.

Change-Id: I398cf8a3513b7fa63c130efc7846a7c5353999d4
Reviewed-on: https://go-review.googlesource.com/37459
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>