Implement python iteration over the operation/region/block hierarchy.

* Removes the half-completed prior attempt at region/block mutation in favor of new approach to ownership.
* Will re-add mutation more correctly in a follow-on.
* Eliminates the detached state on blocks and regions, simplifying the ownership hierarchy.
* Adds both iterator and index based access at each level.

Differential Revision: https://reviews.llvm.org/D87982