feat(core): add zero-copy Deref bridge to bitcoin::Script

[alexanderwiederin]

closes #20

Summary

Implements Deref<Target = bitcoin::Script> for ScriptPubkey and ScriptPubkeyRef, behind a new bitcoin feature flag. The Deref implementation uses a new function, script_as_bytes, which captures the raw pointer passed to the btck_script_pubkey_to_bytes callback and casts it to a &[u8] slice, avoiding any allocation.

Motivation

Silent payments block scanning requires script introspection such as is_p2tr (reference). Delegating this functionality to rust-bitcoin keeps the kernel C API focused on consensus, avoiding scope creep. Further, integrating ScriptPubkey with rust-bitcoin's Script type, provides a suite of new functionality with Bitcoin protocol awareness.

Usage

use bitcoinkernel::ScriptPubkey;

let bytes = hex::decode("5120deadbeefdeadbeefdeadbeefdeadbeefdeadbeefdeadbeefdeadbeefdeadbeef").unwrap();
let script = ScriptPubkey::new(&bytes).unwrap();

// the following functionality is exposed through `bitcoin::Script`
assert!(script.is_p2tr());
assert!(script.witness_version().is_some());
assert!(!script.is_op_return());
println!("{}", script.to_asm_string());

What you should know

Given that Deref is intended to be used for wrappers and their underlying type, the use here may be considered misplaced - bitcoin::Script is not an underlying type, but a peer type from a separate crate that shares the same underlying representation. The alternative would be to add a ScriptPubkey::as_bitcoin_script() method to make the relationship explicit, at the cost of call-site ergonomics. I recognise this is a design decision that requires careful consideration. I'm happy to go either way.

See the Rust documentation on when to implement Deref.