libsyntax: Implement deriving for enums with N-ary variants. r=brson

src/test/run-pass/deriving-via-extension-enum.rs

@@ -0,0 +1,15 @@
+#[deriving_eq]
+enum Foo {
+    Bar(int, int),
+    Baz(float, float)
+}
+
+fn main() {
+    let a = Bar(1, 2);
+    let b = Bar(1, 2);
+    assert a == b;
+    assert !(a != b);
+    assert a.eq(&b);
+    assert !a.ne(&b);
+}
+