Taproot support

[BitcoinComfy]

It would be nice to add Taproot support either with secp256k1-zkp / Trezor code (memory hungry) or implementing Schnorr with existing functions.

[stepansnigirev]

Started working on it.
Required changes:

• schnorr signing (Initial schnorr implementation #25)
• taproot tweaks of private keys
• transaction digest algorithm (bip341)
• psbt support for taproot fields