Bonk is designed for performance and memory efficiency. Defining shapes and performing collision tests require no heap allocations and put no pressure on the garbage collector. If you reuse spatial hashes, Bonk will never cause garbage collection.
Bonk is licensed under the LGPL-3 license. In summary: feel free to include it in your closed-source game and modify it internally at will, but if you make changes that you intend to redistribute, you must freely publish your changes.