provide a directive to bundle ocaml modules

[bobzhang]

the motivation is to help distribute software in some cases, so that I only need throw one file, no need build system for some simple cases

bundle(file1,file2,file3)

---->

module rec File1 : sig #include "file1.mli" end = struct #include "file1.ml" end
....

---

[mjambon]

I don't see how to implement this with a macro, even if cppo supported variadic macros, so okay, it makes sense to have this as a built-in.

I don't have much time at the moment. If you want to contribute it's great. Check out https://github.com/mjambon/documents/blob/master/how-to-contribute.md