Update with latest js_of_ocaml: need review, need testing (could not test myself)

https://github.com/ocsigen/js_of_ocaml/commit/f152eafd86adce5e26a9775e28de0181d5084341 and https://github.com/ocsigen/js_of_ocaml/commit/831f15039e55229b04e2444c417ce42b27ab7a53 introduced `strict mode`.
in that context, accessing `this` do not reference the global object anymore.
there is a new js value `Js.Unsafe.global` that reference the global object.