type erasure for anonymous structs; full translation pass for index kernels

src/pystencils/nbackend/arrays.py

src/pystencils/nbackend/emission.py

src/pystencils/nbackend/functions.py

src/pystencils/nbackend/kernelcreation/kernelcreation.py

src/pystencils/nbackend/kernelcreation/transformations.py

src/pystencils/nbackend/types/basic_types.py

tests/nbackend/test_basic_printing.py

tests/nbackend/types/test_types.py