Simplify format and document related macros

[alexcrichton]

This follows from the discussion in #9012.

• All macros are now defined in terms of format_args! allowing for removal of a good bit of code in the syntax extension
• The syntax extension is now in a more aptly-named file, format.rs
• Documentation was added for the format!-related macros.

[huonw]

Are the sufficiently bad that the expansion should happen in ext::format? i.e. have all the separate expand_format, expand_write etc, with the implementation (with a macro to avoid mega-repetition, presumably):

fn expand_format(args...) {
    let expr = <expr for `::std::fmt::rt::format`>;

    expand_ifmt(Some(expr), args...);
}
// ... expand_write etc.

fn expand_ifmt(call_func: Option<@expr>, ...) {
   // if `call_func` is None parse a function as the first arg,
   // otherwise use `call_func` for the final `expr_call` in `to_expr`
}

The FIXME could then go there.

If this is going to replace fmt! then we should make it properly useable, and, frankly, the errors from inside macro expansion are pretty much useless: no information about the original invocation site at all. (Is #5794 a more appropriate issue for this FIXME?)

[alexcrichton]

@huonw and I think that this is blocked in its current state on #5794. I'll look into fixing that soon, otherwise I think that this perhaps just shouldn't be accepted because it makes certain kinds of errors 10x worse.