Diff from rust-bitcoin and conceptual differences from rust-elements

[apoelstra]

Ideally we could maintain this crate as a fork of rust-bitcoin with some stuff added (and a minimal amount of stuff removed), similar to what we do with e.g. secp256k1-zkp and rust-secp256k1-zkp and elements-miniscript. However there are some big differences in Elements that make this hard.

This issue is to try to exhaustively list all of these. Let me start:

• Network is different in Bitcoin and Elements (Bitcoin has a small fixed list of main/testnet/regtest/signet; Elements has an open-ended "just name a chain" model with some special cases)
• Address has two more encodings, blech32 and blech32m, which are used for confidential addresses
• Script has new opcodes (this actually is probably purely additive and easy to maintain)
• Simplicity support as an extra tapleaf version not present in Bitcoin; Simplicity itself as a whole nother script engine
• Amount is a "confidential amount" which has very different properties
• New TxOut fields: nonce, asset
• TxOut witnesses (surjection proof and rangeproof)
• New TxIn fields (issuances, reissuances and pegins)
• psbt and pset have entirely different interfaces. (psbtv2 is similar, but we still might want to create our own separate crate with blinding support)
• sighash module computation are different for taproot/ecsda and legacy transactions.
• Basic transaction APIs related to blinding/unblinding support.
• Extra blinding key APIs from slip77
• Different BlockHeader and block format with dynafed stuff.

[sanket1729]

I don't think opcodes are additive. For example, if OP_VAULT is added into bitcoin, I think it would replace Checksigfromstack in elements!