The reason I didn't add that is that it isn't stack safe, if the tree is unbalanced, and I had a bug in the balancing logic earlier, so `depth` was actually stack overflowing. :)

I think we should just make `depthExceeds` private to scodec. I was mostly using it for testing, to make sure the trees being built were balanced. It's kind of an implementation detail anyway - for instance, in the future we could change `BitVector` to use some other purely functional data structure that isn't just a balanced tree and that doesn't have a single well-defined notion of depth.