* 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
* Providing stable, C-accessible definitions for bridging MLIR Python<->C APIs, we eliminate inter-extension dependencies (i.e. they can all share a diamond dependency on the MLIR C-API).
* Just provides accessors for context and module right now.
* Needed in NPComp in ~a week or so for high level Torch APIs.
Differential Revision: https://reviews.llvm.org/D88426