Extract ice40_unlut pass from ice40_opt.

Extract ice40_unlut pass from ice40_opt.

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.