Make Rust's `write!` faster

[dtolnay]

This crate should not be necessary. It should be possible to optimize write!(&mut buf, "{}", value) to do what this crate does.

[jhpratt]

Four and a half years on, has there been any discussion of this among the relevant team (libs, presumably)?.

Could this crate be included as a dependency of std, in the same way that hashbrown & others are?