[mlir][Python] Add missing capsule->module and Context.create_module.

* Extends Context/Operation interning to cover Module as well.
* Implements Module.context, Attribute.context, Type.context, and Location.context back-references (facilitated testing and also on the TODO list).
* Adds method to create an empty Module.
* Discovered missing in npcomp.

Differential Revision: https://reviews.llvm.org/D89294