Merge branch 'fhennig/sycl' into 'backend-rework'

docs/source/api/kernelcreation/index.rst

docs/source/backend/ast.rst

docs/source/backend/extensions.rst

docs/source/backend/index.rst

mypy.ini

src/pystencils/__init__.py

src/pystencils/backend/__init__.py

src/pystencils/backend/ast/expressions.py

src/pystencils/backend/emission.py

src/pystencils/backend/exceptions.py

src/pystencils/backend/extensions/__init__.py

src/pystencils/backend/extensions/cpp.py

src/pystencils/backend/extensions/foreign_ast.py

src/pystencils/backend/jit/__init__.py

src/pystencils/backend/jit/gpu_cupy.py

src/pystencils/backend/jit/jit.py

src/pystencils/backend/kernelcreation/__init__.py

src/pystencils/backend/kernelcreation/cpu_optimization.py

src/pystencils/backend/kernelcreation/iteration_space.py

src/pystencils/backend/kernelfunction.py