Shared UTXO model based on stateful tap-tree

[motorina0]

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While I was working on the taproot functionality, a couple of ideas came to me. One of them is for a Shared UTXO model that uses a Stateful Tap-Tree. I'm not sure if this approach was covered before or not. I have described the concept in this gist. I would really like to hear your opinion on it: https://gist.github.com/motorina0/a1da5e8892594aaab3f80422d105e305

cc: @junderw @brandonblack @OttoAllmendinger @tiero @louisinger