Use variant based methods for gpio

stm32f3 provides methods, with which we can access each variant
of a register, without providing offets ourselfs.

This also removes a lot off `unsafe` calls, which where needed before
to access a whole register without any bound guards for fields and
flags.

This makes the macro section quite more verbose,
because we have no method yet, where the index number is not part of the
method name, but rather could be indexed via .e.g. array-field access:

    moder.moder().modify(|_, w| w.moder[$i].alternate());

Author: Sh3Rm4n