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.
Speed up Speed up hb_set_digest_common_bits_t calcs

Correctly this time.
Add hb_set_digest_combiner_t
Add comment
Templatize hb_set_digest_lowest_bits_t filter
Improve hb_set_digest_t

Make Amiri rendering faster a whopping 45% again!  Speends up pretty
much anything I tested.
Remove hb_set_digest_common_bits_t

Was unused.
Remove HB_DEBUG_SET_DIGESTS

Wasn't correct with the new combiner.  I should add it back somehow,
but for now the digests seem to be working very well...
Obssesive optimization

Not measurable by any means, but conceptually this is faster since
the mask matches more often than the digest.
[util] Unbreak --show-text / --show-unicode
Minor
Hand-code bsearch in the hot inner loop.

Saves another 3 / 4 percent with Amiri.
0.9.16
Remove unneeded code

We always push a pause at the end such that each lookup falls in exactly
one pause_map_t.  Now, only if I can find a better name for that...
Minor
Rename "pause" to "stage"

The compile() function is starting to become illegible...
Minor renaming
Minor