[MLton] Added completion to esml-mlb-mode

[Matthew Fluet]

> > With regards to forceUsed, remember that it does not mean to mark every
> > identifier encountered in the body of the annotation as used; instead, it
> > means to mark as used all identifiers in scope at the end of the 
> > annotation as used.
> > 
> > In fact, ffiStr is somewhat the same -- it "does something" at the end of 
> > the annotation scope, rather than during the elaboration of the body of 
> > the annotation.
> > 
> > That difference in behavior is why it might make sense to leave their
> > syntax as "action" annotations rather than "toggle" annotations.
> 
> Fair enough.  Although I don't find it difficult to read a forceUsed
> toggle as saying either "do" or "do not" mark everthing in scope at
> the end of the annotation as used.

I'd be o.k. with "forceUsed {false|true}" since other don't see it in a 
different light.