[MLton] Vector len passed to MakeFormalsRel

[Stephen Weeks]

> Here is a short snippet of output from model-elimination running my
> flattener. The numbers are the lengths of the vectors passed to
> MakeFormalsRel.
...
> MakeFormalsRel 3656

That's a lot of arguments to one function!  Conservatively, that could
take about 80M (= 3656^2 * 1/2 * 12) of space for one equivalence
relation.  This certainly looks like the problem.

After the upcoming release, I'll fix equivalence-relation.fun.

But even with that fix, flatten all is looking like a bad idea.  3000
arguments is a lot to pass.  I don't wanna know how bad a self compile
would be.