Extend struct support; add lookup freeze and typificaiton; add various tests

src/pystencils/nbackend/emission.py

src/pystencils/nbackend/kernelcreation/context.py

src/pystencils/nbackend/kernelcreation/freeze.py

src/pystencils/nbackend/kernelcreation/options.py

src/pystencils/nbackend/kernelcreation/typification.py

src/pystencils/nbackend/types/basic_types.py

tests/nbackend/__init__.py

tests/nbackend/kernelcreation/__init__.py

tests/nbackend/kernelcreation/test_freeze.py

tests/nbackend/kernelcreation/test_index_kernels.py

tests/nbackend/kernelcreation/test_options.py

tests/nbackend/kernelcreation/test_typification.py

tests/nbackend/types/__init__.py

tests/nbackend/types/test_quick_types.py

tests/nbackend/types/test_types.py