libsyntax: Implement deriving via a syntax extension for the IterBytes trait. r=brson

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

@@ -0,0 +1,16 @@
+#[deriving_iter_bytes]
+enum Foo {
+    Bar(int, char),
+    Baz(char, int)
+}
+
+#[deriving_iter_bytes]
+enum A {
+    B,
+    C,
+    D,
+    E
+}
+
+fn main(){}
+

src/test/run-pass/deriving-via-extension-iter-bytes-struct.rs

@@ -0,0 +1,10 @@
+#[deriving_iter_bytes]
+struct Foo {
+    x: int,
+    y: int,
+    z: int
+}
+
+fn main() {}
+
+