[MLton] mlton-lib documentation

[Stephen Weeks]

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

true.

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

Actually, I was thinking that (a subset of) LIST, VECTOR, and ARRAY
almost certainly will be exported.  The argument ordering alone makes
them a huge win over the basis library.  And it will be nice to have
the consistency of argument ordering with HASH_SET and others.