Extensions and fixes to the type system

src/pystencils/__init__.py

src/pystencils/backend/kernelcreation/freeze.py

src/pystencils/sympyextensions/typed_sympy.py

src/pystencils/types/parsing.py

src/pystencils/types/types.py

tests/nbackend/kernelcreation/test_freeze.py

tests/nbackend/types/test_types.py

tests/symbolics/test_typed_sympy.py