Add methods to check idle, tx empty and rx not empty for serial

[azerupi]

There is currently already a method pending_event that allows to get events when the interrupts are enabled. But the idle flag and the txe and rxne flags can also be used when interrupts are disabled (idleie=0, txeie=0 and rxneie=0 respectively).

This PR adds methods on Rx and Tx to read the state of those flags and reset the idle flag. This is inspired by https://github.com/stm32-rs/stm32f1xx-hal/blob/master/src/serial.rs#L303-L324

These methods are also more intuitive IMHO than the pending_event method.

We tested is_idle and clear_idle_interrupt and confirmed that it worked.
What do you guys think?

[hannobraun]

Thank you, @azerupi, looks good to me!