Move io/register.hh -> io/types/register.hh
Move Port-class into io/types/port.hh
Move io/port.hh -> io/types/pin.hh
Migrate io/null.hh inro io/types/{register,port,pin}.hh
Move gpio internal registers to io/registers/port.hh
Add 'mask' variable to Port-classes (unused, TBD)