serial.rs: support for additional interrupts

[joao404]

Hello,

At the moment serial only supports interrupts for bits rxe, txe and idle. There is also a function to check if transmission is complete (is_tx_complete) but no function to enable listening for it. Is anyone currently adding this functionality or is it planned to switch to the same interface as stm32f3xx-hal where there are no specific functions for each event but instead everything is addressed by an enum? I would like to add this in the current fashion, if there are no other plans.