Multi-shot continuations gone forever?

Yep, this initial version has all of the usual caveats of Obj.clone_continuation. I have been an avid user of Obj.clone_continuation for my own research, so my primary interest is to bring it back so I can continue my work. Hopefully as we get experience programming with multi-shot continuations in an environment like OCaml, we can converge towards a more sensible design.