USART: Missing events

[tmplt]

Copied from the internal issue tracker. By @martinmortsell:

The current implementation of the USART peripheral only supports at subset of the available interrupt sources.

pub enum Event {
    RxReady,
    TxReady,
    Overrun,
    TxEmpty,
}

The HAL should support more (all?) Event sources.
In particular, I was going to trigger an interrupt on NSSE in SPI mode.

[tmplt]

I think we'll need to replace the usart::Event with one that takes usart::{spi,uart}::Events:

pub enum Event {
    Spi(spi::Event),
    Uart(uart::Event),
}

and derive strum::FromRepr on the latter. The generics::events::EventHandler implementation will probably have to query the hardware in order to return the correct usart::Event member.