CROP: use C implementations for previously converted asm routines.

a1blit
a2blit
blit8
cblock
cdot
chli
clipit
closestc
copy_bytes
copy_structure
copy_words
cvli
exchange_words
fcuncomp
stuff_words
unbrun
unlccomp
unrun
unsbsrsccomp
xorblock
CROP: switch to C version of conv_screen.
CROP: convert bsame.
CROP: convert fsame.
CROP: convert bcontrast, fixing a bug when all elements are different.

Fixes a bug when all 'count' elements in the two arrays are different.
The original asm did not distinguish between exhausting the count and
failing the repeat condition.
CROP: convert bcompare.
CROP: convert fcompare.
CROP: switch to C version of til_next_skip (tnskip).
CROP: switch to C version of til_next_same (tnsame).
Merge pull request #12 from wangds/crop_c

Convert CROP ASM to C.