Should be improved for hb_bytes_t.

Not enforcing it using type hierarchy.

We cannot rely on constexpr functions...

lvalue iterators must declare __item_type__ as a reference.

Ugly, but does the job.

hb_array_t is its own iterator...

Fixes https://github.com/harfbuzz/harfbuzz/issues/1502

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

In lieu of constructor removed in previous commit.

Ouch!