To fix older compilers again (this was the case in hb_array_t).

hb-ot-layout-common.hh:1353: note: candidate 2: operator[](T*, int) <built-in>
hb-ot-layout-common.hh:1354: error: ISO C++ says that these are ambiguous, even though the worst conversion for the first is better than the worst conversion for the second:
hb-iter.hh:63: note: candidate 1: Item& hb_iter_t<Iter, Item>::operator[](unsigned int) const [with Iter = hb_array_t<const OT::IntType<short unsigned int, 2u> >, Item = const OT::IntType<short unsigned int, 2u>]
hb-ot-layout-common.hh:1354: note: candidate 2: operator[](T*, int) <built-in>
hb-ot-layout-common.hh: In member function 'bool OT::ClassDef::serialize(hb_serialize_context_t*, hb_array_t<const OT::IntType<short unsigned int, 2u> >, hb_array_t<const OT::IntType<short unsigned int, 2u> >)':
hb-ot-layout-common.hh:1490: error: ISO C++ says that these are ambiguous, even though the worst conversion for the first is better than the worst conversion for the second:
hb-iter.hh:63: note: candidate 1: Item& hb_iter_t<Iter, Item>::operator[](unsigned int) const [with Iter = hb_array_t<const OT::IntType<short unsigned int, 2u> >, Item = const OT::IntType<short unsigned int, 2u>]
hb-ot-layout-common.hh:1490: note: candidate 2: operator[](T*, int) <built-in>

hb_array_t is its own iterator...

Should only be done by subclass.

This reverts commit 969ff3c7.

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

Sorry for the noise, downstream custom builders.  Please adjust.

Fixes https://github.com/harfbuzz/harfbuzz/issues/1127

This reverts commit 66920a6b.

Some of our bots (Oracle Studio and Apple gcc 4.2) do not allow
explicit except for constructors.

https://github.com/harfbuzz/harfbuzz/issues/1127

We would need to write:

  for (Iter<T> it (...); bool (t); t++)

instead of:

  for (Iter<T> it (...); t; t++)

But I think it's an improvement in code readability in the long term.

Fixes https://github.com/harfbuzz/harfbuzz/issues/1127

Unused so far.