[mlir][Python] Return and accept OpView for all functions.

* All functions that return an Operation now return an OpView.
* All functions that accept an Operation now accept an _OperationBase, which both Operation and OpView extend and can resolve to the backing Operation.
* Moves user-facing instance methods from Operation -> _OperationBase so that both can have the same API.
* Concretely, this means that if there are custom op classes defined (i.e. in Python), any iteration or creation will return the appropriate instance (i.e. if you get/create an std.addf, you will get an instance of the mlir.dialects.std.AddFOp class, getting full access to any custom API it exposes).
* Refactors all __eq__ methods after realizing the proper way to do this for _OperationBase.

Differential Revision: https://reviews.llvm.org/D90584