Added Marsaglia's original MWC()/CMWC()

The versions in SUPRKISS.vim were especially tuned to avoid
the need for a bigger integer type when computing the carry;
this is the original idea, mostly to see how disgusting it could be to
fake unsigned 64 bit operations in vimscript.

The funny thing is that in the default configuration CMWC() is 2x faster
than MWC() -- that's because of the particular choice of multipliers,
thus showing that the slowest thing is faking uint64_t -- Q.E.D.