Currently, `ice40_opt -unlut` would map SB_LUT4 to $lut and convert
them back to logic immediately. This is not desirable if the goal
is to operate on $lut cells. If this is desirable, the same result
as `ice40_opt -unlut` can be achieved by running simplemap and opt
after ice40_unlut.
o Not all derived methods were marked 'override', but it is a great
feature of C++11 that we should make use of.
o While at it: touched header files got a -*- c++ -*- for emacs to
provide support for that language.
o use YS_OVERRIDE for all override keywords (though we should probably
use the plain keyword going forward now that C++11 is established)