rtlil: Improve performance of SigSpec::extract(SigSpec, SigSpec*)

rtlil: Improve performance of SigSpec::extract(SigSpec, SigSpec*)

Converting to a pool<SigBit> is fairly expensive due to inserts somewhat
frequently causing rehashing.  Instead, walk through the pattern SigSpec
directly on a chunk-by-chunk basis and apply it to this SigSpec's
individual bits.  Using chunks for the pattern minimizes the number of
iterations in the outer loop.