[mlir][python] improve usability of Python affine construct bindings

- Provide the operator overloads for constructing (semi-)affine expressions in
  Python by combining existing expressions with constants.
- Make AffineExpr, AffineMap and IntegerSet hashable in Python.
- Expose the AffineExpr composition functionality.

Reviewed By: gysit, aoyal

Differential Revision: https://reviews.llvm.org/D113010