Elements Hardfork Wishlist

Just for record keeping a wishlist of hardfork changes that might be considered for Liquid v2.

- remove `CHECKSIGFROMSTACK` (keep `CHECKSIGFROMSTACKVERIFY`) (#202)
- Fix #473: " Non-witness transactions cannot include issuances"
- Taproot script with threshhold support in wallet (SF actually, but hey it's a lit)
- Bulletproof rangeproofs, including generalized sidechannel in witness data
- Make block header CProof witness data stack of bytes not script https://github.com/ElementsProject/elements/issues/404
- Fix #259: "reissuance tokens can not be exercised from unblinded outputs"
- `ComputeWitnessHash` should, like in Core, compute the wtxid instead of withash
- relax MAX_MONEY constraint

2022-02-10:
- more sighash flags on txes with version 3 or higher (or segwit versions 2 or higher, or whatever)