UTF8 decoding

Benchmark: timing 10000 iterations of decode-UTF-8, decode-utf8, open-UTF-8, open-utf8, ptiny...
decode-UTF-8: 20 wallclock secs (19.94 usr +  0.13 sys = 20.07 CPU) @ 498.26/s (n=10000)
decode-utf8: 20 wallclock secs (19.77 usr +  0.18 sys = 19.95 CPU) @ 501.25/s (n=10000)
open-UTF-8: 25 wallclock secs (24.95 usr +  0.16 sys = 25.11 CPU) @ 398.25/s (n=10000)
 open-utf8:  4 wallclock secs ( 3.16 usr +  0.15 sys =  3.31 CPU) @ 3021.15/s (n=10000)
     ptiny: 25 wallclock secs (25.24 usr +  0.12 sys = 25.36 CPU) @ 394.32/s (n=10000)
               Rate       ptiny  open-UTF-8 decode-UTF-8 decode-utf8   open-utf8
ptiny         394/s          --         -1%         -21%        -21%        -87%
open-UTF-8    398/s          1%          --         -20%        -21%        -87%
decode-UTF-8  498/s         26%         25%           --         -1%        -84%
decode-utf8   501/s         27%         26%           1%          --        -83%
open-utf8    3021/s        666%        659%         506%        503%          --