Better lower bounds

[sharmaeklavya2]

Currently, we use ceil(area) as a lower bound on the minimum number of bins. It's easy to come up with inputs where ceil(area) is far from optimal (but I don't know how true this is for 'interesting' inputs). We need something better.

These lower-bounds should either be very efficiently computable, or we should have a mechanism where a user can choose to compute them on-demand-only.

[sharmaeklavya2]

I have added more lower-bounds. They transform the width and height of items by applying dual-feasible-functions and then ceil(area) of this transformed instance gives us lower-bounds.