[mlir] Expose MemRef layout in Python bindings

This wasn't possible before because there was no support for affine expressions
as maps. Now that this support is available, provide the mechanism for
constructing maps with a layout and inspecting it.

Rework the `get` method on MemRefType in Python to avoid needing an explicit
memory space or layout map. Remove the `get_num_maps`, it is too low-level,
using the length of the now-avaiable pseudo-list of layout maps is more
pythonic.

Depends On D94297

Reviewed By: mehdi_amini

Differential Revision: https://reviews.llvm.org/D94302