I'm going to close this: it's a really nice idea, but after the 30-bit 
long digits were implemented, the speedup doesn't seem to be worth the 
extra code complication.

The only situation I've found where this optimization really does make a 
big difference is when using 60-bit digits, but allowing those in Python 
would take a bit more work (essentially because it requires using some 
inline assembler to get at the CPU widening multiply and 128-bit-by-64-bit 
division instructions).