Un-32-bit-ify bignum.lisp in various ways, even trying to
do a quick fixup on the comments to remove 32-bit
assumptions. Hasn't been tested with a real 64-bit
implementation, mind you.
There's a *lot* of code for doing bignum digit divides
(anything prefixed with #!+32x16-divide); no platform
currently uses it, but it might be worth twiddling with
the code to see if it's an improvement over the
VOP currently implementing %FLOOR. Certainly it'd be
nice to move more code into Lisp-land.