Fix is_subtype_of - we cannot use ->equals

Fix is_subtype_of - we cannot use ->equals

Calling $parent->equals in is_subtype_of is broken. The parent may be a union
type, in which case it overrides $parent->equals will never return true.

However, union types override ->is_a_type_of and ->is_subtype_of to do
something sensible. Changing $parent->equals to $parent->is_a_type_of respects
that overriding and gets us sane behavior.