writeln support

[pdgilbert]

It would be nice to have support for writeln! ( which comes with "use core::fmt::Write;" ). I think this is done in stm32f1xx-hal and stm32f4xx-hal by implementing a trait. I'm a newbie, so probably wrong about how it is done, but I do know it works with those two hals and not with stmf3xx-hal.

[teskje]

Looks like they simply implement core::fmt::Write for their serial tx type: https://github.com/stm32-rs/stm32f4xx-hal/blob/master/src/serial.rs#L1763

We have such a type as well, just implemented inside a macro (Tx<$USARTX>). It should be very easy to impl Write for it too.