Wrap OCaml functions before passing them to JavaScript

[vouillon]

This is necessary to deal with the calling conventoin mismatch introduced by the upcoming effect support (ocsigen/js_of_ocaml#1340)

This partially addresses #42.

This was done rapidly, so I may have missed a few places.

I'm not sure what to do with Brr.Ev.listen/Brr.Ev.unlisten since removeEventListenerexpects to be invoked with the very same JavaScript function that was passed to addEventListener.

[dbuenzli]

Thanks Jérôme. Very much appreciated, saved me quite a bit of time to fix this.

I have merged your patch as b14c3f9. The follow up patch c64b00b tweaks it, hunts a few remaining cases that needed wrapping and updates the FFI cookbook.