[LOWPRI] Unify class "instance types" with anonymous object types.

There is a totally different syntax for anonymous object types and class instance types:
(Ptyp_object and Pcty_sig respectively). One is used to define types of object without knowing their classes, and the other is to define the return value of a constructor ("instance types"). They are unified by the type system seamlessly, but their type definitions use a different syntax.
I'm in the middle of sending out a parser update that improves them independently but they are not unified yet. There's no reason why they can't be unified, as the Ptyp_object syntactically only includes methods, whereas Pcty_sig includes that and much more. Just one more thing that unnecessarily gives objects a bad reputation.

Start by reviewing the parser update mentioned in the issue and the definitions of Ptyp_object and Pcty_sig. Determine how their syntax can be unified while preserving anonymous object types and constructor instance types; done means both use the unified syntax and the type system continues to handle them seamlessly.