So it could be inlined.

Using bit-tricks it could be implemented without condition
(improved trick version by Minux Ma).

Simple benchmark shows it is faster on i386 and x86_64, though
I don't know will it be faster on other architectures?

benchmark                       old ns/op     new ns/op     delta
BenchmarkFastrand-3             2.79          1.48          -46.95%
BenchmarkFastrandHashiter-3     25.9          24.9          -3.86%

Change-Id: Ie2eb6d0f598c0bb5fac7f6ad0f8b5e3eddaa361b
Reviewed-on: https://go-review.googlesource.com/34782Reviewed-by: Minux Ma <minux@golang.org>
Run-TryBot: Minux Ma <minux@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>