Holes don't work

[jonsterling]

Try the following code:

def _ : univ := ?

The result is some error:

dune exec algaett test.ag
Fatal error: exception Invalid_argument("app_ulvl")
Raised at Stdlib__Effect.Deep.try_with.(fun) in file "effect.ml", line 69, characters 47-54
Called from Loader.load in file "src/loader/Loader.ml", line 17, characters 21-78
Called from Dune__exe__Main in file "src/bin/Main.ml", line 3, characters 9-41

I never ran the code that builds holes, and this code is itself very subtle (in part b/c of the handling of universe polymorphism, which we are hitting here). So this should be debugged.