When we unify two variables that end up merged, the rank of the
resulting content is the lower of the two variables being merged. But
during storage, we really do mean, take the target descriptor of the
type we're merging against, and don't try to lower to a
possibly-generalized rank! This fixes a couple bugs I didn't even
realize were present!
This enables you to write something like
```
A := U8
List.map [1, 2, 3] @A
```
which will be compiled as if it was `List.map [1, 2, 3] \x -> @A x`.
Closes#3499
