strings: use Index in Count

strings: use Index in Count

This simplifies code and provides performance iprovments:
Similar to https://go-review.googlesource.com/#/c/28577

CountHard1-48               1.74ms ±14%  0.17ms ±14%  -90.16%  (p=0.000 n=19+19)
CountHard2-48               1.78ms ±15%  0.25ms ±13%  -86.10%  (p=0.000 n=19+20)
CountHard3-48               1.78ms ±12%  0.80ms ±11%  -55.19%  (p=0.000 n=17+20)
CountTorture-48             13.5µs ±14%  13.6µs ±11%     ~     (p=0.625 n=18+19)
CountTortureOverlapping-48  6.92ms ±13%  8.42ms ±11%  +21.72%  (p=0.000 n=19+17)

Change-Id: Ief120aee918a66487c76be56e0796871c8502f89
Reviewed-on: https://go-review.googlesource.com/28586
Run-TryBot: Ilya Tocar <ilya.tocar@intel.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>