make borrowed_bitsets record how many bits were desired
make borrowed_bitsets record how many bits were desired

This is a modest win for speed (zero-resetting),
and a modest win for correctness (catching reads/writes
     that should be invalid; luckily it caught no bugs!)

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