Switch most dispatches to NQP-based dispatcher.

Next up will be hunting/fixing regressions.
Allow binding to $/.

Because we need it now to fix other things, but also because it will
fix an RT once this branch is merged.
Fix $/ setting and $_ lookup a bit.

We "got lucky" because the callframe of the proto was magically going
missing, thanks to an optimization. This should resolve it, for now at
least, though it feels like we're missing a more real solution.