todo: add runtime support when ce cannot be found

Still need to collect and then generate ZEND_VERIFY_VARIANCE opcodes

Known issues:
  - When there is an inheritance warning it gets generated twice.
  - The VERIFY_VARIANCE opcode is not generated in the correct place

Refactor a bit of variance verification compilation.

Fix bad error message

Fixes this under opcache:

class A {
  function m(Traversable $z) {}
}

class B extends A {
  function m(iterable $z) {}
}

Also fixes cases where A::m is Iterator, IteratorAggregate, etc.

Fixes this under opcache:

class A {
  function m(): iterable {}
}

class B extends A {
  function m(): Traversable {}
}

Also fixes cases where B::m is Iterator, IteratorAggregate, etc.