Add equality

equality/main.pony

@@ -0,0 +1,54 @@
+actor Main
+    new create(env: Env) =>
+        // identity equality: there is only one None
+        // so None `is` None
+        if None is None then
+            env.out.print("None is None")
+        end
+
+        let a = Foo("hi")
+        let b = Foo("hi")
+
+        // a and b are different objects
+        if a is b then
+            env.out.print("a is b")
+        else
+            env.out.print("a is not b")
+        end
+
+        let c = a
+
+        // c is a reference to the same object to which
+        // a is a reference
+        // I assume? I don't actually know how the memory
+        // management works lol
+        if a is c then
+            env.out.print("a is c")
+        else
+            env.out.print("a is not c")
+        end
+
+        // structural equality: see fun eq() below
+        if a == b then
+            env.out.print("a is equal to b")
+        end
+        if a == c then
+            env.out.print("a is equal to c")
+        end
+
+class Foo
+    let field: String
+
+    new create(field': String) =>
+        field = field'
+
+    // the value returned by this function implements
+    // structural equality
+    fun eq(that: box->Foo): Bool =>
+        this.field == that.field
+
+
+// Extra note about primitives: since all primitives have no fields and there
+// is only one instance of each user-defined primitive, all primitives of a
+// given type are equal by identity and structurally equal. (I'm not sure
+// about builtin primitives.)