Support js_of_ocaml and possibly melange

[polytypic]

Multicore magic is not really interesting per se for use from js_of_ocaml or melange, but there are potentially cases where a library mainly written with the native compiler in mind would be used via those. In such a case it would be nice then that multicore-magic would basically just work also in that case. So, multicore-magic should ideally be tested and potentially changed to also work with js_of_ocaml and possibly with melange. Looking at how melange treats record types, for example, it is clear that multicore-magic does not currently work with it, but it would likely be possible to implement suitable work arounds.