Add RegisterBlock trait

[burrbull]

r? @therealprof

cc @TeXitoi

[burrbull]

Rebased.

cc @therealprof

We need this if we want to have generic implementation of some code (without macros), but Deref is unavailable for some reason. For example when we split USARTx on Tx and Rx.

[burrbull]

The idea was proposed by @TeXitoi and instead of using macro let us to write something like

impl<USART> Tx<USART> where USART: RegisterBlock<RB=pac::usart1::RegisterBlock> {
    pub fn listen(&mut self) {
        unsafe { (*USART::rb()).cr1.modify(|_, w| w.txeie().set_bit()) };
    }
}