[Bitcoin-ml] Bucketed UTXO Commitment part 2

[Tomas]

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