Support taproot script-path spendings

[crisdut]

Description

Currently, the TapTree::merkle_root is not implemented. This causes error during transfer creation step in RGB v0.11:

RGB: command-line wallet for RGB smart contracts
     by LNP/BP Standards Association

Loading descriptor from wallet default ... success
Loading stock ... success
thread 'main' panicked at 'not yet implemented', /bp-std/derive/src/taptree.rs:144:9
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

Type: bug Version: https://github.com/BP-WG/bp-std/commit/a2005978df1078c3ea55c11db27693c9cd5a9052

[dr-orlovsky]

Partially closed in 4a234b94136613f1f5ef8d409fed2829f0d8e346

[crisdut]

I'm going to close this issue, okay?

[dr-orlovsky]

Nope - script part is not yet implemented.

[St333p]

Does this issue also cover the addition of a tapret commitment to a non-empty taptree, in which a nonce is used to ensure unique position of the tapret leaf as described in LNPBP12?

[dr-orlovsky]

@St333p Basically this issue is because there is no taproot implementation for script path spendings. So this is unrelated to RGB - it is just the fact that RGB when tries to build taproot tree fails (since it is not yet supported)