Incorrect error position for implicit instantiation

[gebner]

Errors from instantiate_implicits are shown at unrelated locations:

```pulse // error shown here too!
fn foo
  (n : nat) // error shown here: Expected expression of type Type got expression n of type nat
  requires emp
  returns m:nat
  ensures pure (m == n)
{
  id #n // <- but the issue is here
}