Merge bitcoindevkit#1031: remove duplicate is_empty

f5074ee remove duplicate `is_empty` from DerivationAdditions (Vladimir Fomene)

Pull request description:

  Duplicate `is_empty` method

  ### Checklists

  #### All Submissions:

  * [x] I've signed all my commits
  * [x] I followed the [contribution guidelines](https://github.com/bitcoindevkit/bdk/blob/master/CONTRIBUTING.md)
  * [x] I ran `cargo fmt` and `cargo clippy` before committing

ACKs for top commit:
  danielabrozzoni:
    ACK f5074ee

Tree-SHA512: 1cb48d25c9e57dbe444646ebc33c838e3d58c2523d74b58a75f0d085f68d070ec6a644c46e0bccb8765af7167c8359f079a6222fc9d17ae28ce1e6dda820fc84

crates/chain/src/keychain.rs

@@ -46,11 +46,6 @@ pub use txout_index::*;
 pub struct DerivationAdditions<K>(pub BTreeMap<K, u32>);
 
 impl<K> DerivationAdditions<K> {
-    /// Returns whether the additions are empty.
-    pub fn is_empty(&self) -> bool {
-        self.0.is_empty()
-    }
-
     /// Get the inner map of the keychain to its new derivation index.
     pub fn as_inner(&self) -> &BTreeMap<K, u32> {
         &self.0
@@ -72,6 +67,7 @@ impl<K: Ord> Append for DerivationAdditions<K> {
         self.0.append(&mut other.0);
     }
 
+    /// Returns whether the additions are empty.
     fn is_empty(&self) -> bool {
         self.0.is_empty()
     }

crates/chain/tests/test_keychain_txout_index.rs

@@ -5,6 +5,7 @@ mod common;
 use bdk_chain::{
     collections::BTreeMap,
     keychain::{DerivationAdditions, KeychainTxOutIndex},
+    Append,
 };
 
 use bitcoin::{secp256k1::Secp256k1, OutPoint, Script, Transaction, TxOut};