constant elimination, part 2: subexpression extraction

src/pystencils/backend/ast/structural.py

src/pystencils/backend/ast/util.py

src/pystencils/backend/kernelcreation/iteration_space.py

src/pystencils/backend/kernelcreation/typification.py

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

src/pystencils/kernelcreation.py