Don't store interleaved_bits_.
Don't store interleaved_bits_.

This saves 24 bytes per zbox, and seems to have a 1-2% performance
boost (testing with stepped_pools).

Lasercake/Lasercake,elidupree/Lasercake,elidupree/Lasercake,elidupree/Lasercake,idupree/Lasercake,Lasercake/Lasercake,Lasercake/Lasercake,idupree/Lasercake,idupree/Lasercake,idupree/Lasercake,elidupree/Lasercake,elidupree/Lasercake,Lasercake/Lasercake,idupree/Lasercake,idupree/Lasercake,elidupree/Lasercake,idupree/Lasercake,Lasercake/Lasercake,elidupree/Lasercake