make C stubs non allocating?

[avsm]

I notice that the C stubs do a range check and then raise an exception if it's over. Instead of that, we could always mask off the index with page_size, and so overflow back to the start

This would make the rest of the stub non-allocating and safe to mark noalloc, and be a minor speedup if its on the critical path. Needs to be benchmarked though.