hslib/Word.hs: memoize power-of-2-minus-1 masks for integer wraparound
hslib/Word.hs: memoize power-of-2-minus-1 masks for integer wraparound

cuts memory allocation for 10k iterations from 1.93G to 1.24G

surprisingly, this change is a wash as far as total execution time..

mit-pdos/fscq-impl,mit-pdos/fscq-impl,mit-pdos/fscq-impl,mit-pdos/fscq-impl,mit-pdos/fscq-impl