[Bitcoin-ml] Bucketed UTXO Commitment part 2

I have updated and started implementing the bucketed UTXO commitment approach. Most notable change, is that I now propose to use a variable depth for the prefix tree and a fixed maximum bucket size. This absolves the need for full recalculation when the bucket count changes, which as Chris Pacia noted is rather annoying. The result is also considerably simpler. The spec, code and results can be found here: https://github.com/tomasvdw/bips/blob/master/BIP-UtxoCommitBucket.mediawiki Still some work is needed to optimize the EC multiset implementation. Most notably the current method for normalizing a multiset to hash is inefficient, but I think the results are quite promising as there is enough wiggle room for improvement. Tomas van der Wansem bitcrust