Sizeless vectorization

Surprisingly easy follow-up to !232 (merged) to support sizeless ARM SVE and RISC-V V. It uses some ugly hacks to sneak C functions like svcntb() into places that expect Python integers. Python duck-typing and SymPy made it possible. Not sure whether this should be merged as-is, but making it nicer would require re-writing CBackend. At least I couldn't think of a better way to obtain the innermost loop counter and loop stop.