make register reader/writer copiable, return writer

[burrbull]

cc @rust-embedded/tools
Do anyone see disadvantage of using value instead of mutable reference for writer?

Related to #708

Big breaking change.

This does not let you create writer outside of write as suggested by #708, but lets to reuse of writer.

for example
this

apb.rstr().modify(|_, w| w.$perrst().set_bit());
apb.rstr().modify(|_, w| w.$perrst().clear_bit());

can be optimized to

let w = apb.rstr().modify(|_, w| w.$perrst().set_bit());
apb.rstr().write(|_| w.$perrst().clear_bit());

R<REG> -> W<REG> conversion could be added also. Unsafe?

[thejpster]

That seems like you could get yourself into trouble (accidentally using a w from the function scope not the closure argument, or saving a w and not using for a call, then using it again after and losing your changes).

Is there any evidence that what we have is inefficient?