message: Add void and unit types to prelude
diff: @@ -96,6 +96,26 @@ let vcons : fun (Elem : _) -> fun (len : _) -> Elem -> Vec Elem len -> Vec Elem
  cons _ head (tail Vec nil cons);
  +// Void
  +
  +let Void : Type
  + = fun (Void : Type) -> Void;
  +
  +let absurd : fun (A : Type) -> Void -> A
  + = fun A => fun void => void A;
  +
  +
  +// Unit
  +
  +let Unit : Type
  + = fun (Unit : Type) ->
  + fun (unit : Unit) ->
  + Unit;
  +
  +let unit : Unit
  + = fun Unit => fun unit => unit;
  +
  +
  // Leibniz equality
  // This states that two things of the same type are equal if every property of
  