[bdk_chain_redesign] Introduce Append trait for additions

Before, we were using `core::ops::AddAsign` but it was not the most
appropriate.

crates/chain/src/keychain.rs

@@ -14,14 +14,13 @@
 //! [`KeychainChangeSet`]s.
 //!
 //! [`SpkTxOutIndex`]: crate::SpkTxOutIndex
-use core::ops::AddAssign;
 
 use crate::{
     chain_graph::{self, ChainGraph},
     collections::BTreeMap,
     sparse_chain::ChainPosition,
     tx_graph::TxGraph,
-    ForEachTxOut,
+    Append, ForEachTxOut,
 };
 
 #[cfg(feature = "miniscript")]
@@ -71,12 +70,12 @@ impl<K> DerivationAdditions<K> {
     }
 }
 
-impl<K: Ord> DerivationAdditions<K> {
+impl<K: Ord> Append for DerivationAdditions<K> {
     /// Append another [`DerivationAdditions`] into self.
     ///
     /// If the keychain already exists, increase the index when the other's index > self's index.
     /// If the keychain did not exist, append the new keychain.
-    pub fn append(&mut self, mut other: Self) {
+    fn append(&mut self, mut other: Self) {
         self.0.iter_mut().for_each(|(key, index)| {
             if let Some(other_index) = other.0.remove(key) {
                 *index = other_index.max(*index);
@@ -87,12 +86,6 @@ impl<K: Ord> DerivationAdditions<K> {
     }
 }
 
-impl<K: Ord> AddAssign for DerivationAdditions<K> {
-    fn add_assign(&mut self, rhs: Self) {
-        self.append(rhs)
-    }
-}
-
 impl<K> Default for DerivationAdditions<K> {
     fn default() -> Self {
         Self(Default::default())