Speedup stringsearch for two byte strings

Speedup stringsearch for two byte strings

Uses the lower byte with memchr which is
significantly faster than a naive compare

Performance difference with bench (http://hastebin.com/xuxexataso.js):

old                             new

single character                single character
Κ found at 922                  Κ found at 922
3324                            616
㎡ found at 13217               ㎡ found at 13217
42366                           4931
က found at 4096                 က found at 4096
13369                           9836
＀ found at 65280                ＀ found at 65280
207472                          36149
ﾥ found at 65445                ﾥ found at 65445
209344                          36666
  found at 8197                   found at 8197
26731                           11757
倂 found at 20482               倂 found at 20482
66071                           17193

linear search                   linear search
ΚΛ found at 922                 ΚΛ found at 922
4112                            504
㎡㎢ found at 13217             ㎡㎢ found at 13217
55105                           5119
ﾥﾦ found at 65445               ﾥﾦ found at 65445
268016                          35496

linear + bmh search             linear + bmh search
ΚΛΜΝΞΟΠΡ found at 922           ΚΛΜΝΞΟΠΡ found at 922
2897                            522
ﾥﾦﾧﾨﾩﾪﾫﾬ found at 65445         ﾥﾦﾧﾨﾩﾪﾫﾬ found at 65445
167687                          158465

Review URL: https://codereview.chromium.org/1303033012

Cr-Commit-Position: refs/heads/master@{#30587}