bytes, strings: optimize Split*

bytes, strings: optimize Split*

The relevant benchmark results on linux/amd64:

bytes:

SplitSingleByteSeparator-4   25.7ms ± 5%   9.1ms ± 4%  -64.40%  (p=0.000 n=10+10)
SplitMultiByteSeparator-4    13.8ms ±20%   4.3ms ± 8%  -69.23%  (p=0.000 n=10+10)
SplitNSingleByteSeparator-4  1.88µs ± 9%  0.88µs ± 4%  -53.25%  (p=0.000 n=10+10)
SplitNMultiByteSeparator-4   4.83µs ±10%  1.32µs ± 9%  -72.65%  (p=0.000 n=10+10)

strings:

name                         old time/op  new time/op  delta
SplitSingleByteSeparator-4   21.4ms ± 8%   8.5ms ± 5%  -60.19%  (p=0.000 n=10+10)
SplitMultiByteSeparator-4    13.2ms ± 9%   3.9ms ± 4%  -70.29%  (p=0.000 n=10+10)
SplitNSingleByteSeparator-4  1.54µs ± 5%  0.75µs ± 7%  -51.21%  (p=0.000 n=10+10)
SplitNMultiByteSeparator-4   3.57µs ± 8%  1.01µs ±11%  -71.76%  (p=0.000 n=10+10)

Fixes #18973

Change-Id: Ie4bc010c6cc389001e72eab530497c81e5b26f34
Reviewed-on: https://go-review.googlesource.com/36510
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>