Reorganized ExtCore- and FSharp.Compatibility.OCaml-related functions so it's easier to see what should (and will) be moved into those libraries, and what is NHol-specific.
Added the [<Experimental>] attribute to Ratio.normalize_ratio so we know where it's being used (because it doesn't actually do anything at the moment, and so could be the source of some errors).
Added some OPTIMIZE comments for later.