core: make the public fmt API completely safe.

[eddyb]

No description provided.

[eddyb]

The usecase I have for this is a macro like:

#![feature(macro_rules)]

macro_rules! format_lite {
    ($($piece:expr)#+ $(,$arg:expr)+) => {
        ::std::fmt::format(&::std::fmt::Arguments::new(&[$($piece),*],
            &[$(::std::fmt::argument(::std::fmt::Show::fmt, &$arg)),*]))
    }
}

fn main() {
    std::io::println(&*format_lite!("foo"#"bar", 5u));
}

[alexcrichton]

I'm not 100% certain that the check of if self.formatter as uint == Argument::show_uint as uint { means that this is actually safe, so I'd like to mull this over a bit longer.

[brson]

These appear to be breaking changes.