Print error message in #[panic_implementation]

[geofft]

It should be

#[lang = "panic_fmt"]
extern fn panic_fmt(msg: fmt::Arguments, file: &'static str, line: u32, col: u32) -> !;

and possibly also an #[unwind] or #[unwind(allowed)] in future definitions of Rust. See libcore's definition and longer discussion in #19 (comment)

[alex]

Updated the title to reflect that with #[panic_implementation] we now have the right signature, but we still don't print the message.