Library formatting

Decided

D-0073, settled by the language’s author: part of spec §18. Older text cites it as 18.73.

No global format: String::format(fmt, args: ..&Format, allocator = …) builds a String, chosen by the user over a global function and over a macro, which waits for M10. Format is a real trait: std implements it for the numbers, bool, char, str, cstr and String, and a type of one’s own implements it to be formatted; a Formatter appends to a String and reports the placeholder’s spec, which a type’s own implementation interprets. No attribute marks a format string: an #[format] was built and removed by the user, who wants formatting functions of one’s own to be macros, as in Rust — String::format is the one function whose format string the compiler checks, until M10. println and friends stay builtins, taking only the built-in kinds, until macros (M10); error(…) keeps its static storage (§9.2). Revised by 18.80: M10 has come, and String::format and the builtins are the macros format! and println!