Implement format instructions

[paupino]

The following code:

let a = Decimal::new(100, 2);
let b = Decimal::new(300, 2);
let c = a / b;
println!("{}", c);
println!("{:.2}", c);

produces the output:

0.3333333333333333333333333333
0.

This is because the implementation converts to a string first and applies the pad method. Decimal should implement further format instructions to support producing 2dp without requiring rounding first up.