[MLton] DisjointSet name changes

[Stephen Weeks]

> I  always  really  feel  annoyed  by  !  because it is so easy to miss or not
> recognize.  Perhaps it is my eyes and the small fonts I use finally  catching
> up with me.
> 
> Any  way,  either  way  is  ok  with  me,  although I am sure that I will get
> confused a bit when I see := and assume that the LHS is a ref.

I suspect the typical use will be as qualified identifiers, so you
will see Set.! and Set.:=.   I'm not sure that's so beautiful,
though.

My main reason for doing the change is for uniformity, since the
operations have exactly the same semantics as for refs.  In fact, one
can think of disjoint sets as a generalization of refs with an
additional "union" operation.