[mlir] Expose Value hierarchy to Python bindings

Values are ubiquitous in the IR, in particular block argument and operation
results are Values. Define Python classes for BlockArgument, OpResult and their
common ancestor Value. Define pseudo-container classes for lists of block
arguments and operation results, and use these containers to access the
corresponding values in blocks and operations.

Differential Revision: https://reviews.llvm.org/D89778