function stringSource(s) {
            var i=0; return function() {
                return i < s.length ? s.charCodeAt(i++) : null;
            };
        }
Creates a source function for a string.
@param {string} s String to read from
@returns {function():number|null} Source function returning the next char code respectively `null` if there are
 no more characters left.
@throws {TypeError} If the argument is invalid
@inner