New UTF-8 stride function

[Juan Manuel Cabo]

And these are the results for the same linux 64bit system but 
compiling with -m32:

$ dmd -m32 -O -inline -release -noboundscheck fast_stride.d

$ for a in *wiki*; do echo ; echo $a: ; ./fast_stride $a; done

arwiki-latest-all-titles-in-ns0:
stride 89362
myStride 49974
myStride 51140
stride 88308

dewiki-latest-all-titles-in-ns0:
stride 138381
myStride 116971
myStride 116662
stride 139681

enwiki-latest-all-titles-in-ns0:
stride 584787
myStride 490681
myStride 490909
stride 584694

ruwiki-latest-all-titles-in-ns0:
stride 585372
myStride 333905
myStride 341274
stride 585050

--jm