[MLton] -warn-unused true and MLton

[Matthew Fluet]

> Matthew, since you're looking into -sequence-unit true on the MLton
> sources, I'll look into -warn-unused true.  I'll skip the x86-codegen,
> which has 928 of the 2288 warnings, for now if you want to look at
> those.

O.k.  In a likewise move, I'm skipping the elaborator stuff, which has
about a dozen errors.

I've been changing:

(f x; ()) ==> ignore (f x)

(f x; e) ==> let val _ = f x
             in e
             end

(f x; g y; ()) ==> let val _ = f x
                       val _ = g y
                   in ()
                   end

Probably the single most common -sequence-unit error is
ignoring the result of DirectedGraph.addEdge.