Make core::fmt::Void a non-empty type.

Because we handle artifically-constructed references to it in live code which
is a totally broken thing to do.

src/libcore/fmt/mod.rs

@@ -166,7 +166,9 @@ pub struct Formatter<'a> {
 // NB. Argument is essentially an optimized partially applied formatting function,
 // equivalent to `exists T.(&T, fn(&T, &mut Formatter) -> Result`.
 
-enum Void {}
+struct Void {
+    _priv: (),
+}
 
 /// This struct represents the generic "argument" which is taken by the Xprintf
 /// family of functions. It contains a function to format the given value. At