Implement back end for SAFA

[sga001]

• Add support for SAFA in backend
• Update cost model to consider only 1 combined table. Only thing cost model needs to decide is batch size (maybe k stride stuff?).

[ecmargo]

K stride is decided before hand so the cost model can consider it but not really
Is the polynomial eval just going away now?

[sga001]

Yes, poly eval is goooooone. Because when we merge tables it doesn't make sense to have it.