Merge branch 'backend-rework' into fhennig/unique-types

src/pystencils/backend/ast/expressions.py

src/pystencils/backend/ast/logical_expressions.py

src/pystencils/backend/emission.py

src/pystencils/backend/kernelcreation/ast_factory.py

src/pystencils/backend/kernelcreation/freeze.py

src/pystencils/backend/kernelcreation/iteration_space.py

src/pystencils/backend/kernelcreation/typification.py

src/pystencils/backend/platforms/generic_gpu.py

src/pystencils/backend/transformations/__init__.py

src/pystencils/backend/transformations/eliminate_branches.py

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

tests/nbackend/kernelcreation/test_freeze.py

tests/nbackend/kernelcreation/test_typification.py

tests/nbackend/test_code_printing.py

tests/nbackend/transformations/test_branch_elimination.py

tests/nbackend/transformations/test_constant_elimination.py