This is in preparation for switching to the Core standard library.

Currently, the compiler relies on the presence of the Js and Belt modules for quite a few things, for example async/await (Js.Promise.unsafe_async/Js.Promise.unsafe_await) or array/list spread (Belt.List.concatMany/Belt.Array.concatMany).

You can find more such usages by searching for Lident "Js" or Lident "Belt" in the compiler sources.

This should be refactored so that functions from the runtime modules (caml_*) are used instead.