function fail(c) {
                    throw Error("Illegal character code: "+c);
                }
Decodes base64 char codes to bytes.
@param {!function():number|null} src Characters source as a function returning the next char code respectively
 `null` if there are no more characters left.
@param {!function(number)} dst Bytes destination as a function successively called with the next byte.
@throws {Error} If a character code is invalid