return raw value from write/modify

[burrbull]

This looks not so beautiful like #738, but almost not breaking and allows to transfer data between writes with unsafe.

    let bits = rcc.apb1rstr().modify(|_, w| w.usart2rst().clear_bit());
    rcc.apb1rstr().write(|w| unsafe { // save on 1 read operation
        w.bits(bits).usart2rst().set_bit()
    });

[Emilgardis]

Isn't this a huge change?

[burrbull]

@Emilgardis let's merge this one too.

[Emilgardis]

Hmm, I guess it shouldn't break to much, since people should have closed the expression with ;

[burrbull]

Hmm, I guess it shouldn't break to much, since people should have closed the expression with ;

Compared to #738 this does not affect on expressions inside write, only outside.
For example in f1xx-hal there is only 18 places that require adding ;.