move integer_functions and fast_approximations into sympyextensions module

src/pystencils/backend/kernelcreation/freeze.py

src/pystencils/boundaries/inkernel.py

src/pystencils/sympyextensions/astnodes.py

src/pystencils/sympyextensions/fast_approximation.py

src/pystencils/sympyextensions/integer_functions.py

src/pystencils/sympyextensions/math.py

src/pystencils/timeloop.py

tests/test_fast_approximation.py

tests/test_math_functions.py

tests/test_sympyextensions.py