Various extensions to the vectorizer

src/pystencils/backend/platforms/generic_cpu.py

src/pystencils/backend/platforms/x86.py

src/pystencils/backend/transformations/ast_vectorizer.py

...pystencils/backend/transformations/eliminate_constants.py

src/pystencils/backend/transformations/select_intrinsics.py

src/pystencils/codegen/target.py

tests/fixtures.py

tests/kernelcreation/test_functions.py

tests/kernelcreation/test_type_cast.py

tests/nbackend/transformations/test_ast_vectorizer.py