Revert "Speed up hb_set_digest_common_bits_t calcs"

This reverts commit 3d1b66a35e1ab3be19335705f310b278d76d66d2.

The calculations were buggy.  It's not worth optimizing right now.
Print set-digest hit ratio with HB_DEBUG_SET_DIGESTS
Initialize set digests

We were not initializing the digests properly and as a result they were
being initialized to zero, making digest1 to never do any useful work.

Speeds up Amiri shaping significantly.