Use a better random instance generator for bin-packing

[sharmaeklavya2]

Issues with the current random instance generator (bp1):

• Many 1-by-1 items are present, which make the game easy.
• opt(I) is usually ceil(area(I)).

Ideally we would like small number of items that are hard to pack optimally. Maybe tweaking some parameters of bp1 would improve it, but I doubt it. bp1 was written to be easy to implement, and I didn't do any research on how to generate hard inputs.

[sharmaeklavya2]

The generator guill outputs instances that can be hard, so this issue is kind-of solved, but I'm still looking for more generators to get more variety.