Fix `costIsAcceptable`

[effectfully]

costIsAcceptable in both PIR and UPLC thinks that any Constr node containing a single element is fine as long as the cost of the latter is acceptable:

costIsAcceptable :: Term name uni fun a -> Bool
costIsAcceptable = \case
  <...>
  Constr _ _ es ->
    case es of
      []  -> True
      [e] -> costIsAcceptable e
      _   -> False

Which makes sense on the surface until you realize that you can a 100-level-deep Constr node each containing a single element at each level -- and the cost of that isn't acceptable, because all those levels are gonna get evaluated at runtime. This needs to be fixed.