Fiddled w/ /F's cool new splitbins function: documented it, generalized it

Fiddled w/ /F's cool new splitbins function:  documented it, generalized it
a bit, sped it a lot primarily by removing the unused assumption that None was
a legit bin entry (the function doesn't really need to assume that there's
anything special about 0), added an optional "trace" argument, and in __debug__
mode added exhaustive verification that the decomposition is both correct and
doesn't overstep any array bounds (which wasn't obvious to me from staring at the
generated C code -- now I feel safe!).  Did not commit a new unicodedata_db.h, as
the one produced by this version is identical to the one already checked in.