Speed up Speed up hb_set_digest_common_bits_t calcs

Correctly this time.
Add hb_set_digest_combiner_t
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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.