Module display

Enables base64'd output anywhere you might use a Display implementation, like a format string.

use base64::{display::Base64Display, engine::general_purpose::STANDARD};

let data = vec![0x0, 0x1, 0x2, 0x3];
let wrapper = Base64Display::new(&data, &STANDARD);

assert_eq!("base64: AAECAw==", format!("base64: {}", wrapper));