Started refactor

src/pystencils/__init__.py

src/pystencils/backend/__init__.py

src/pystencils/backend/arrays.py

src/pystencils/backend/ast/__init__.py

src/pystencils/backend/ast/collectors.py

src/pystencils/backend/ast/dispatcher.py

src/pystencils/backend/ast/kernelfunction.py

src/pystencils/backend/ast/nodes.py

src/pystencils/backend/ast/transformations.py

src/pystencils/backend/ast/tree_iteration.py

src/pystencils/backend/ast/util.py

src/pystencils/backend/constraints.py

src/pystencils/backend/emission.py

src/pystencils/backend/exceptions.py

src/pystencils/backend/functions.py

src/pystencils/backend/jit/__init__.py

src/pystencils/backend/jit/cpu_extension_module.py

src/pystencils/backend/jit/jit.py

src/pystencils/backend/kernelcreation/__init__.py

src/pystencils/backend/kernelcreation/analysis.py