Fixed the generate_panic macro.

This now just passes all the arguments to panic directly. This appears
to work well with or without the Rust standard library.

src/macros.rs

@@ -6,7 +6,7 @@ macro_rules! generate_panic
 {
    ($($arg: tt)+) =>
    ({
-      panic!(::std::fmt::format(format_args!($($arg)+)));
+      panic!($($arg)+);
    })
 }