64-bit performance

We should have pointers of a size intermediate between 32 and 64 bits, let's say 40-bit
pointers. The point being that the it'd be large enough to address the RAM necessary but
doesn't waste so much space.

I really don't think we'll ever grow to the point where we'll use all of the 64-bit pointer
address space, and pointers with top 20-30 bits unused are just wasted space.

Too bad that the whole world thinks in 2^n.