Use bitshift-based Rec.word_updN
Use bitshift-based Rec.word_updN

This is definitely using unproven code, but it's really satisfying to see the
~30x performance improvement it provides for mkfs. The `split1` function in
Word.hs is expensive, and the performance increase comes mostly from removing
the 32768 calls to it.

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