[MLton] mlton-lib documentation

[Matthew Fluet]

> I think it may be better to strive for consistency in
> naming conventions and signatures without any automated support.  The
> SML Basis Library shows that that kind of consistency isn't too hard.

I agree.  One thing that I have found annoying about lib/mlton (which
would be somewhat alleviated by includes) is functions missing in similar
structures.  For example, LIST has:

      val foreach: 'a t * ('a -> unit) -> unit
      val foreachi: 'a t * (int * 'a -> unit) -> unit
      val foreach2: 'a t * 'b t * ('a * 'b -> unit) -> unit

while VECTOR has:

      val foreach: 'a t * ('a -> unit) -> unit
      val foreachi: 'a t * (int * 'a -> unit) -> unit
      val foreach2: 'a t * 'b t * ('a * 'b -> unit) -> unit
      val foreachi2: 'a t * 'b t * (int * 'a * 'b -> unit) -> unit
      val foreach3: 'a t * 'b t * 'c t * ('a * 'b * 'c -> unit) -> unit

If we had a FOREACH_SEQ signature, then we could keep them in sync.

Of course, LIST and VECTOR aren't likely to be candidates for exporting to
the world, so it's probably a moot point.