Add initial implementation for horizontal reductions for vectorization

src/pystencils/backend/ast/vector.py

src/pystencils/backend/emission/ir_printer.py

src/pystencils/backend/kernelcreation/typification.py

src/pystencils/backend/platforms/x86.py

src/pystencils/backend/transformations/loop_vectorizer.py

src/pystencils/backend/transformations/select_intrinsics.py

tests/kernelcreation/test_reduction.py