Implement a scan macro

[nrc]

Should be easier to implement in ezio than in the general case because we don't need to worry about error handling or optimal performance. Something like the following should work:

// Types should not always be required.
let (foo, bar): (u8, f64) = scanln!("{}, {}");

alternative syntaxes:

let foo: u8;
let bar: f64;
scanln!("{}, {}", foo, bar);

let (foo, bar) = scanln!("{}, {}", u8, f64);

There are many, many ways this could be enhanced following format strings.