BUGFIX-66

Bug 22: Noncontiguous Subtraction (Not Quite)

This is useful, e.g., for subtracting the interleaved coordinates of a Morton space-filling curve.
This code computes from minus what, but only for bit positions where mask is 1.
If bit i of mask is 0 then bit i of the return value is bit i of from (for 0 <= i < 64).
If bit i of mask is 1 then bit i of the return value is bit i of from, minus bit i of what, minus borrow.
Borrows are generated as usual and they propagate (unchanged) through bit positions where mask is 0.