Support half-duplex serial

[llamington]

Currently the serial function requires a tuple of RX/TX pins. The reference manual states that "the TX and RX lines are internally connected", so only one pin would be required as an argument.

[burrbull]

You mean both read and write on TX line?

[llamington]

Yes that's correct

[burrbull]

Not implemented yet.

It's better to create independent SerialHalfDuplex structure for this with new::(pin: TX) and its own methods.
As I understand it will be semantically incompatible with read/write from embedded-hal.

[bluespider42]

@llamington Did you ever make any progress on this?

[llamington]

@bluespider42 no, unfortunately I didn't (I ended up using the STM32HAL library in C)