Parses a bare key.

def _parse_bare_key(self):  # type: () -> Key
        """
        Parses a bare key.
        """
        key_type = None
        dotted = False

        self.mark()
        while self._current.is_bare_key_char() and self.inc():
            pass

        key = self.extract()

        if self._current == ".":
            self.inc()
            dotted = True
            key += "." + self._parse_key().as_string()
            key_type = KeyType.Bare

        return Key(key, key_type, "", dotted)