Part two is solved by repeatedly removing all free rolls until there
are no more rolls to remove. When removing the rolls we keep a running
count of how many rolls we've removed.
We look at each valid (having coordinates inside the grid) neighbor to
a cell including a paper roll and then see if the neighbor contains a
roll of paper or not. Finally we count the number of rolls, if it is
less than 4 we can pick it.
The solution I went with is to walk the entire bank of batteries with
their joltages and pair them up. At every step I can either form an
incomplete candidate (Left x), or a complete candidate (Right (a, b))
if I already have a previous candidate.
