type_object: remove layout and base type from PyTypeInfo

[davidhewitt]

This PR removes some associated types from PyTypeInfo which are really just implementation details of #[pyclass]:

• Layout: moved to PyClassImpl. This doesn't make sense in the long term anyway, because if we inherit from native types where we don't know the layout information (Extend Python Class from Rust #991, Support subclassing from native types in abi3 #1344) we need #[pyclass] to work without layout information.
• BaseType - moved to PyClassImpl. Not all Python types have a base type (e.g. PyAny), or we might not statically know them (e.g. Extend Python Class from Rust #991). All #[pyclass] do have a base type and we'll always know what it is.
• BaseLayout - removed entirely.
• Intitializer - moved to PyClassBaseType (which is renamed from PyBaseTypeUtils).

Making this change frees us up to make internal implementation changes to eventually support #991 and #1344.

In addition, it also simplifies a lot of trait bounds dotted around the public interface and simplifies the macros in types/mod.rs.

[messense]

LGTM.

[davidhewitt]

(I'm going to add some more tests before merging) EDIT: the lines which codecov reports as not covered are all trait bounds and stuff like that which can't be covered anyway. Strange.