add foreign_ast and CppMethodCall. Add proper index translation for SYCL.

src/pystencils/__init__.py

src/pystencils/backend/emission.py

src/pystencils/backend/extensions/__init__.py

src/pystencils/backend/extensions/cpp.py

src/pystencils/backend/extensions/foreign_ast.py

src/pystencils/backend/platforms/sycl.py