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.

This reverts commit 3d1b66a3.

The calculations were buggy.  It's not worth optimizing right now.

Fixes test-set.c

Fails now.  Fixing.

Implement two set digests, and one that combines the two.

This was causing all object types to be non-POD and have static
initializers.  We don't need that!

Now, most nil objects just moved from .bss to .data.  Fixing for that
coming soon.

They can be macros on some systems.  Eg. mingw32.