This adds a sneaky unification between the expression of the function
return value in the unification. I am not entirely sure how often this
will get used, but it could be valuable in the right instance if this
isn't already learned through other sources. I'm fairly confident that
it isn't incorrect, so in the worst case scenario it's redundant
information for the unification solver.
This is being added as a separate commit so that it's obvious how this
type of unification invariant can be applied.
0f30f47249