Fatal if a class implements IteratorAggregate and Iterator

It doesn't make sense to implement both IteratorAggregate and Iterator.
Which of those two are supposed to be iterated? PHP fatals on this, we should too.

Part of #3536
This might cause issue for people using `extends PDOStatement implements IteratorAggregate`
But since it wouldn't do what you expect it to do anyway (See #3536 why PdoStatement is broken), we should tell them

Test Plan:
`test/run test/slow/ext_class`