Macro format_in 

macro_rules! format_in {
    (in $alloc:expr, $($arg:tt)*) => { ... };
}

A std::format!-style constructor for the arena String.

use css_parse::{Arena, format_in};
let alloc = Arena::default();
let str = format_in!(in &alloc, "{}px", 12);
assert_eq!(str.as_str(), "12px");