Add failing (on C backend) test for deep struct equality.

eg/equality.castile

@@ -0,0 +1,15 @@
+struct name { first: string; last: string }
+struct person { age: integer; name: name }
+
+fun main() {
+  a = 40 as string|integer
+  b = 40 as string|integer
+  if a == b {
+    print("these union-type values are equal")
+  }
+  j = make person(age: 23, name:make name(first:"Bamber", last:"Smith"));
+  k = make person(age: 23, name:make name(first:"Bamber", last:"Smith"));
+  if j == k {
+    print("these deep struct-type values are equal")
+  }
+}

tests/Castile.md

@@ -807,6 +807,26 @@ Structs of two different types cannot be tested for equality.
     | }
     ? mismatch
 
+Deeply nested structs can be tested for equality.
+
+    | struct name { first: string; last: string }
+    | struct person { age: integer; name: name }
+    | main = fun() {
+    |   j = make person(age: 23, name:make name(first:"Bamber", last:"Smith"));
+    |   k = make person(age: 23, name:make name(first:"Bamber", last:"Smith"));
+    |   j == k
+    | }
+    = True
+
+    | struct name { first: string; last: string }
+    | struct person { age: integer; name: name }
+    | main = fun() {
+    |   j = make person(age: 23, name:make name(first:"Bamber", last:"Smith"));
+    |   k = make person(age: 23, name:make name(first:"Amber", last:"Smith"));
+    |   j != k
+    | }
+    = True
+
 Structs cannot be compared for ordering.
 
     | struct person { age: integer; name: string }