Per-word utf8_to_bytes()

This changes utf8_to_bytes() to do a per-word initial scan to see if the
source is actually downgradable, before starting the conversion.  This
is significantly faster than the current per-character scan.  However,
the speed advantage evaporates in doing the actual conversion to being a
wash with the previous scheme.

Thus it finds out quicker if the source is downgradable.

cache grind yields this, based on a 100K character string; the
non-downgradable one has the next character after that be the only one
that's too large.:

Key:
    Ir   Instruction read
    Dr   Data read
    Dw   Data write
    COND conditional branches
    IND  indirect branches
    _m   branch predict miss
    _m1  level 1 cache miss
    _mm  last cache (e.g. L3) miss
    -    indeterminate percentage (e.g. 1/0)

The numbers represent relative counts per loop iteration, compared to
blead at 100.0%.
Higher is better: for example, using half as many instructions gives 200%,
while using twice as many gives 50%.

unicode::bytes_to_utf8_legal_API_test
Downgrading 100K valid characters

        blead   proposed
       ------ ------
    Ir 100.00  99.99
    Dr 100.00 100.03
    Dw 100.00 100.04
  COND 100.00 100.05
   IND 100.00 100.00

COND_m 100.00  87.25
 IND_m 100.00 100.00

 Ir_m1 100.00 123.25
 Dr_m1 100.00 100.18
 Dw_m1 100.00  99.94

 Ir_mm 100.00 100.00
 Dr_mm 100.00 100.00
 Dw_mm 100.00 100.00

unicode::bytes_to_utf8_illegal
Finding too high a character after 100K valid ones

        blead   fast
       ------ ------
    Ir 100.00 188.91
    Dr 100.00 179.77
    Dw 100.00  66.75
  COND 100.00 278.47
   IND 100.00 100.00

COND_m 100.00  88.71
 IND_m 100.00 100.00

 Ir_m1 100.00 121.86
 Dr_m1 100.00 100.01
 Dw_m1 100.00 100.03

 Ir_mm 100.00 100.00
 Dr_mm 100.00 100.00
 Dw_mm 100.00 100.00

0 files changed, 0 insertions, 0 deletions