It's as good as it gets, and seems to be on par with previous set implementation
in my benchmark.

Would be great if someone can double-check my bitops.

With new set implementation, this became really costy.  Optimize it.
There's more to be done, but this shaves off most of the fat.

Part of fixing https://bugs.chromium.org/p/chromium/issues/detail?id=782220

Fixes https://github.com/behdad/harfbuzz/issues/579.

Ouch!  That's what happens when one plays with increment/decrement operators!

Fixes https://github.com/behdad/harfbuzz/issues/578

Fixes https://github.com/behdad/harfbuzz/issues/577

Fixes https://github.com/behdad/harfbuzz/issues/576

c:\projects\harfbuzz\src\hb-set-private.hh(151): error C2327: 'hb_set_t::page_t::v': is not a type name, static, or enumerator [C:\projects\harfbuzz\build\harfbuzz.vcxproj]

Fixes clang "non-const reference cannot bind to vector element" error.

Fixes https://github.com/behdad/harfbuzz/pull/23

This one:

  map->mask = (1 << (next_bit + bits_needed)) - (1 << next_bit);

before the fix, the shift was done as an int, causing overflow
if it ever got to 1 << 31.  Sprinkle 'u's around.

Fixes https://bugs.chromium.org/p/chromium/issues/detail?id=634805

I experimented with replacing use of hb_set_digest_t with this new
hb_frozen_set_t, hoping to get a huge speedup for busy lookups
(like kern lookup in Roboto), but I only got 6% speendup in Roboto
and 4% in NotoNastaliqUrduDraft :(.

In preparation for fixing:
https://code.google.com/p/chromium/issues/detail?id=403594

Fixes previous commit's test.

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

Was unused.

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

Correctly this time.