Make the code more Crystalline

[mattkimmel]

The initial code for this project was copied and pasted directly from https://github.com/mak-it/bin_packing, and then modified as minimally as possible to get tests passing in Crystal.

It would be good to audit the code and do a few things, including but not limited to:

• Changing Ruby idioms to Crystal idioms where possible
• Putting in more explicit typing--the type inference engine is doing a lot of work and the code isn't all that readable
• Fixing up the specs a bit, e.g. should eq true becomes should be_true, and so on

This is probably an ongoing task, but here's an issue to track it.