Implementing the `uses-serial` trait

The uses-serial trait is formally defined by YEP-306.

The uses-serial trait is not typically going to be a terminal trait. More specific configuration will be provided by traits which depend on it. These must also implement the direct_serial_write method. See uses-uart and uses-i2c for more in depth implementation.

built 2020-09-26 00:52:26                                      CC0: no copyright