Faster / more compact code.

Just blacklist based on table sizes like we do for other fonts.

This reverts commit 0c1b287b.

This is much faster then calling a bunch of individual add()'s.

Prevents O(n^2) run times.

This new API returns cvXX and ssXX related NameId, things like
featUiLabelNameId, featUiTooltipTextNameId, sampleTextNameId, ... of cvXX
and UINameId of ssXX, in a unified way.

However HarfBuzz currently doesn't expose an API for retrieving the actual
information associated with NameId from the `name` table and that should be
done separately.

Also, uncomment code again, just "if (0)" it out, so it doesn't get stale again.

Towards possibly using Null pool for some nil objects.

If a pointer type was passed to Null(), reinterpret_cast<> was
complaining about qualifiers being removed. Turns out I need the const on
the other side of "Type" to fix that.  Also remove unused const from
NullPool type.

Not sure if I like to keep it. For now, aim for correctness.

Common Regoin for Access Protection.  Like the NullPool, but writable.

We should hang those off somewhere else. For now, the unused ones can go.

Part of https://github.com/harfbuzz/harfbuzz/issues/1017

If we don't have visibility, make NullPool static.