Previously we would construct the shapes of unions used in the pattern
tree for exhaustiveness checking using the type of the branch patterns,
rather than the type of the condition variable. Clearly we want to
always use the condition variable, otherwise some branches will be
seen as exhaustive, when they are not!
To do this, we now index into the condition variable while refying the
patterns to build the tree for exhaustiveness checking.
Closes#4068
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
