Add support for defining metaclasses in Rust

Declaring a metaclass in Rust requires to extend PyType and to adapt the default new implementation (since a metaclass should always allow to create a class in Python).

This would allow for efficient implementation of __instancecheck__ and __subclasscheck__. It would also allow to keep track of information about created classes in Rust instead of storing them as standard attributes on the class which can be modified later.

I am fine doing the changes. I am however open to suggestions as to where to document this.

Specifying a metaclass when defining a class in Rust is out of scope.

Start by reading PyType and the default new implementation, then review the existing Rust-side class-definition support. Determine where metaclass support should be documented, as the issue leaves that location open. Done means Rust-defined metaclasses can support class creation, instancecheck, and subclasscheck, while specifying a metaclass when defining a class remains out of scope.