code: function stringSource(s) {
          var i=0; return function() {
              return i < s.length ? s.charCodeAt(i++) : null;
          };
      }
docs: 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