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.