BUGFIX-66

Bug 21: Noncontiguous Addition (Nearly)

This is useful, e.g., for adding the interleaved coordinates of a Morton space-filling curve.
This code computes to plus from, 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 to (for 0 <= i < 64).
If bit i of mask is 1 then bit i of the return value is bit i of to, plus bit i of from, plus carry.
Carries are generated as usual and they propagate (unchanged) through bit positions where mask is 0.