Implement `Write` on the serial ports so that it can be easily made into a `println!`

[walksanatora]

please... since I cannot implement Write on your types

[Kage-Yami]

please... since I cannot implement Write on your types

Looks like they do now:

• https://docs.rs/uart_16550/0.3.0/src/uart_16550/port.rs.html#139
• https://docs.rs/uart_16550/0.3.0/src/uart_16550/mmio.rs.html#108

[phip1611]

Fixed via #41