Various fixes to constants

src/pystencils/backend/constants.py

src/pystencils/backend/kernelcreation/typification.py

src/pystencils/types/basic_types.py

tests/nbackend/kernelcreation/test_typification.py

tests/nbackend/test_constant_folding.py

tests/nbackend/test_constants.py

tests/nbackend/types/test_constants.py