fixes to pattern matching and codegen, J example works now

tests/black/equality.newt

@@ -10,3 +10,15 @@ sym Refl = Refl
 
 trans : {A : U} {x y z : A} -> Eq x y -> Eq y z -> Eq x z
 trans Refl Refl = Refl
+
+coerce : {A B : U} -> Eq A B -> A -> B
+coerce Refl a = a
+
+J : {A : U} ->
+    {C : (x y : A) -> Eq x y -> U} ->
+    (c : (x : _) -> C x x Refl) ->
+    (x y : A) ->
+    (p : Eq x y) ->
+    C x y p
+-- this was failing until I constrained scrutinee to the constructor + args
+J c x y Refl = c x