Add a to_bytes iface and a handful of impls

src/libcore/to_bytes.rs

@@ -0,0 +1,19 @@
+iface to_bytes {
+    fn to_bytes() -> ~[u8];
+}
+
+impl of to_bytes for ~[u8] {
+    fn to_bytes() -> ~[u8] { copy self }
+}
+
+impl of to_bytes for @~[u8] {
+    fn to_bytes() -> ~[u8] { copy *self }
+}
+
+impl of to_bytes for str {
+    fn to_bytes() -> ~[u8] { str::bytes(self) }
+}
+
+impl of to_bytes for @str {
+    fn to_bytes() -> ~[u8] { str::bytes(*self) }
+}