[MLton] Evolution idea

[Matthew Fluet]

> Also, it may be useful to detect on option unused function objects in
> sequences:
>
> let ... in f; e end --> error if f is a function object.

There is a comment in elaborate-core.fun that indicates where we could add
a warning for non-unit typed expressions in sequences.  The function case
above would fall out as a special case.  On the down side, it means that
fully applied functions might also be warned, because one might only be
interested in their side-effects, not their results.  But, a few
judiciously placed ignore applications would eliminate that.