Skip-ahead for UTF8 encoding

[nickva]

Just like we do it for ASCII-only input, this should help unicode-heavy input.

Turned out better than expected:

===== With input Encode Twitter =====
Name                                       ips        average  deviation         median         99th %
jiffy (encode-utf8-scan-through)        306.51        3.26 ms    +/-15.30%        3.21 ms        4.31 ms
jiffy (master)                          276.38        3.62 ms    +/-13.79%        3.54 ms        4.58 ms
Comparison:
jiffy (encode-utf8-scan-through)        306.51
jiffy (master)                          276.38 - 1.11x slower +0.36 ms
===== With input Encode UTF-8 escaped =====
Name                                       ips        average  deviation         median         99th %
jiffy (encode-utf8-scan-through)       22.41 K       44.62 us     +/-1.54%       44.49 us       46.63 us
jiffy (master)                         10.12 K       98.78 us     +/-0.75%       98.68 us      100.95 us
Comparison:
jiffy (encode-utf8-scan-through)       22.41 K
jiffy (master)                         10.12 K - 2.21x slower +54.16 us
===== With input Encode UTF-8 unescaped =====
Name                                       ips        average  deviation         median         99th %
jiffy (encode-utf8-scan-through)       23.35 K       42.83 us     +/-1.70%       42.68 us       45.00 us
jiffy (master)                         10.59 K       94.42 us     +/-0.72%       94.30 us       96.42 us
Comparison:
jiffy (encode-utf8-scan-through)       23.35 K
jiffy (master)                         10.59 K - 2.20x slower +51.60 us

[davisp]

+1 Nice find as always!