[MLton] Show type at point / OCaml -dtypes

[Stephen Weeks]

> I haven't yet looked into implementing the feature in detail, but is
> it really necessary to modify the elaborate pass? Would it be possible
> to output the info based on the intermediate language produced by the
> elaborate pass? In other words, does[n't] the intermediate language
> produced by elaborate contain both the exact source region and the exact
> type information?

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