constant elimination, part one: idempotence, dominance, and folding

src/pystencils/backend/ast/__init__.py

src/pystencils/backend/ast/expressions.py

src/pystencils/backend/ast/util.py

src/pystencils/backend/kernelcreation/typification.py

src/pystencils/backend/transformations/__init__.py

...pystencils/backend/transformations/eliminate_constants.py

src/pystencils/kernelcreation.py

tests/nbackend/transformations/test_constant_elimination.py