[MLton] Show type at point / OCaml -dtypes

[Vesa Karvonen]

Quoting Stephen Weeks <sweeks@sweeks.com>:
> The output of the elaborator is CoreML (core-ml/core-ml.sig).  All of
> the region information is gone, except for that needed by the match
> compiler to report errors.

Ok.  I assumed there would be more region info due to the fact that
defunctorize produces error messages.

> Also, the information needed to choose good type constructor names is
> gone.  So, I think the best way to go is modify the elaborator.

Ok.  Sounds reasonable.  I think, I'm going to be examing the MLton
front-end and elaborate passes in more detail in the following days
as time permits.

-Vesa Karvonen