[MLton] sequenceNonUnit

[Stephen Weeks]

> it does print an error/warning for sequence expressions with 
> non-unit types that are later constrained by a signature to be unit.
..
> To fix the warning, you need to put a type annotation on cleanup.  I
> think this behavior is fine, because it would be fairly difficult (I
> believe) to distinguish the above case from one where wind was also
> used within the structure at non-unit type.

Could this be accomplished by delaying the warnings a bit?  Maybe not
a good idea because then they would be out of place with other
messages, but, to pick an extreme, it seems like delaying the warnings
until after elaboration is done would prevent this issue.