Introduction of structural ast nodes

src/pystencils/backend/ast/structural.py

src/pystencils/backend/kernelcreation/freeze.py

src/pystencils/backend/transformations/add_pragmas.py

src/pystencils/backend/transformations/ast_vectorizer.py

src/pystencils/backend/transformations/eliminate_branches.py

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

...ils/backend/transformations/hoist_loop_invariant_decls.py

src/pystencils/backend/transformations/rewrite.py