modularize CPU jit

src/pystencils/codegen/config.py

src/pystencils/jit/cpu/__init__.py

src/pystencils/jit/cpu/cpujit.py

src/pystencils/jit/cpu/cpujit_pybind11.py

src/pystencils/jit/cpu/pybind11_kernel_module.tmpl.cpp

tests/jit/test_cpujit.py