[MLton] deepFlatten exception

[Stephen Weeks]

> The tricky part isn't MLton_eq introduced as part of the
> implementation of MLton_equal (polymorphic equality); the tricky
> part is direct usage of MLton_eq by the user as MLton.eq.  Then it
> isn't so clear to me that it's OK to say they're not equal if we're
> not sure.

Actually, that's the situation I was thinking of.  Just as MLton_eq is
used by MLton_equal to optimize polymorphic equality, I would think
that the only sensible use of MLton.eq by a user is to optimize some
other notion of equality where they don't want to pay the full cost of
polymorphic equality if MLton.eq returns false.

Given that our documentation says "the semantics is a bit murky", I
think we have free reign.  We could add something to the documentation
about the intended usage and that we feel free to conservatively
return false when we're unsure.  We could also ask Joe what he's using
it for, or ask on MLton-user for feedback.

Perhaps this is all moot since we're not going to tweak the
deepFlatten transformation.