Make the maximum array size adjustable

[treeowl]

We currently hard-code 5, 32, 0x1f, etc., throughout the module. It would be better to use top-level constants (or, better, parameterize the package with backpack, however that works). Completely aside from allowing performance experiments, this will allow testing with smaller arrays. Using arrays of length 8 or even 4 would let us test with much deeper trees than we can now.