[MLton] sequenceNonUnit

[Stephen Weeks]

> Thinking about it briefly, I came up with the following
> implementation scheme. During the recursive type inference step, the
> inferred type (which may be a variable) of the "ignored expression"
> is saved to a list (along with the necessary info to give proper
> diagnostics). After type inference, a diagnostic would be given for
> each non-unit type in the list. This way the diagnostic given for
> the latentError example above, and other similar cases, would
> (properly) point to the "ignored expression".
>
> Would this work and capture the intention of sequenceNonUnit?

Yes.  It looks good.