policy: Implement Simplicity key traits for Miniscript equivalents

This is the easiest solution to use Miniscript keys in Simplicity
contexts. We focused on subtraits before, which would require changes to
(Elements) Miniscript, but we can also use generic impl blocks, as long
as they are inside rust-simplicity (orphan rule). We removed
elements-miniscript from rust-simplicity to avoid a cyclic dependency.
MiniscriptKey is defined in rust-miniscript, so importing it here does
not create a cycle. We get a diamond pattern where both
elements-miniscript and rust-simplicity depend on the same library,
which should be okay.

Author: uncomputable

Cargo.toml

@@ -17,6 +17,7 @@ path = "src/lib.rs"
 
 [dependencies]
 bitcoin = { version = "0.30.0", optional = true }
+bitcoin-miniscript = { package = "miniscript", version = "10.0" }
 byteorder = "1.3"
 elements = { version = "0.22.0", optional = true }
 hashes = { package = "bitcoin_hashes", version = "0.12" }

src/policy/key.rs

@@ -1,3 +1,4 @@
+use bitcoin_miniscript::{MiniscriptKey, ToPublicKey};
 use elements::bitcoin::key::XOnlyPublicKey;
 use hashes::sha256;
 use std::fmt::{Debug, Display};
@@ -8,8 +9,8 @@ pub trait SimplicityKey: Clone + Eq + Ord + Debug + Display + std::hash::Hash {
     type Sha256: Clone + Eq + Ord + Display + Debug + std::hash::Hash;
 }
 
-impl SimplicityKey for XOnlyPublicKey {
-    type Sha256 = sha256::Hash;
+impl<Pk: MiniscriptKey> SimplicityKey for Pk {
+    type Sha256 = <Pk as MiniscriptKey>::Sha256;
 }
 
 /// Public key which can be converted to a (x-only) public key which can be used in Simplicity.
@@ -21,13 +22,13 @@ pub trait ToXOnlyPubkey: SimplicityKey {
     fn to_sha256(hash: &Self::Sha256) -> sha256::Hash;
 }
 
-impl ToXOnlyPubkey for XOnlyPublicKey {
+impl<Pk: ToPublicKey> ToXOnlyPubkey for Pk {
     fn to_x_only_pubkey(&self) -> XOnlyPublicKey {
-        *self
+        <Pk as ToPublicKey>::to_x_only_pubkey(self)
     }
 
     fn to_sha256(hash: &Self::Sha256) -> sha256::Hash {
-        *hash
+        <Pk as ToPublicKey>::to_sha256(hash)
     }
 }