Consolidate codegen and JIT modules.

.gitlab-ci.yml

docs/Makefile

docs/source/api/codegen.rst

docs/source/api/field.rst

docs/source/api/jit.rst

docs/source/api/sympyextensions.rst

docs/source/backend/index.rst

docs/source/backend/jit.rst

docs/source/backend/objects.rst

docs/source/index.rst

docs/source/reference/api/index.rst

docs/source/reference/gpu_kernels.md

docs/source/reference/kernelcreation.md

docs/source/tutorials/01_tutorial_getting_started.ipynb

pytest.ini

src/pystencils/__init__.py

src/pystencils/_deprecation.py

src/pystencils/backend/__init__.py

src/pystencils/backend/constraints.py

src/pystencils/backend/emission/base_printer.py