Skip local variables that are optimized out for 'cy exec' (in case user didn't build with Cython's build_ext)

Ensure 'cy locals' and 'cy exec' don't use uninitialized local variables (which fixes a lot of 'cy exec' core dumps! :)

make IntNode smarter about the type of integer it chooses for its value, promote plain >=32bit literals to Python objects to make sure their values pass through the C code