ability to run code and check output in tests

tests/CaseEval.newt

@@ -0,0 +1,32 @@
+module CaseEval
+
+data Nat : U where
+  Z : Nat
+  S : Nat -> Nat
+
+plus : Nat -> Nat -> Nat
+plus Z y = y
+plus (S x) y = S (plus x y)
+
+data Eq : {A : U} -> A -> A -> U where
+  Refl : {A : U} -> {x : A} -> Eq x x
+
+two : Eq (plus (S Z) (S Z)) (S (S Z))
+two = Refl
+
+three : Eq (plus (S Z) (S (S Z))) (plus (S (S Z)) (S Z))
+three = Refl
+
+addZero : {n : Nat} -> Eq (plus Z n) n
+addZero {n} = Refl
+
+infixr 1 _,_
+data Pair : U -> U -> U where
+  _,_ : {A B : U} -> A -> B -> Pair A B
+
+fst : {A B : U} -> Pair A B -> A
+fst (a,b) = a
+
+-- I had an ordering issue, but it didn't show up with only one constructor argument
+test : Eq (fst (Z, S Z)) Z
+test = Refl