Use zend_type inside type lists

Use zend_type inside type lists

Instead of having a completely independent encoding for type list
entries. This is going to use more memory, but I'm not particularly
concerned about that, as type unions that contain multiple classes
should be uncommon. On the other hand, this allows us to treat
top-level types and types inside lists mostly the same.

A new ZEND_TYPE_FOREACH macros allows to transparently treat list
and non-list types the same way. I'm not using it everywhere it could be
used for now, just the places that seemed most obvious.

Of course, this will make any future type system changes much simpler,
as it will not be necessary to duplicate all logic two times.