Feature/Enhancement Request: new `OP_GRAFTROOT`

[k98kurz]

Feature or Enhancement?

• (x) New feature
• ( ) Enhancement of existing feature

Description

In the current development cycle for v0.7.0, I have implemented tool functions generating scripts for graftroot lock, keyspend witness, and surrogate execution witness using tapescript code valid in v0.6.2. I also made tools for "graftap" (graftroot-within-taproot) to reduce the size of the locking script for scenarios in which locking scripts must be concise. Below are the byte code sizes for the resulting scripts:

	Graftroot	Graftaproot
Lock	58	36
Keyspend Unlock	67	66
Scriptspend (Overhead)	69	145

If an OP_GRAFTROOT opcode was implemented, it would reduce the size of a Graftroot locking script to 36 bytes if implemented similarly to the updated OP_TAPROOT syntax planned in #14. The witnesses could also be reduced by 1 byte -- selection of keyspend or surrogate branch was originally done in the locking script, but the dup size push d32 eq condition had an additional overhead of 5 bytes, so I instead made the branch selection in the witnesses (true or false at the end of the witness).

Tasks

• Add OP_GRAFTROOT to VM
• Add OP_GRAFTROOT to compiler and decompiler
• Update tools

Additional context