The ideas there are all part of hb-array.hh now.  To be determined how we
want to use generic iterator patterns.

I think I like to keep this hb_supplier_t thing separately from hb_array_t.

We'll see which old clang versions this breaks...

We cannot take address hb_bytes_t direction.  We need to use the
newly added hb_addressof(), ala std::addressof().

Fixes https://github.com/harfbuzz/harfbuzz/issues/1491

Not needed.  We get it in our config.h automatically thanks to
AC_USE_SYSTEM_EXTENSIONS.  Let's see whose build it breaks...
If we end up putting it back, we should add other things from
that macro and remove the macro.

Related to https://github.com/harfbuzz/harfbuzz/issues/1478

In lieu of constructor removed in previous commit.

Ouch!

Looks like operator hb_array_t<> from vector works here. :)