[bdk_chain_redesign] Introduce ChainOracle and TxIndex traits

The chain oracle keeps track of the best chain, while the transaction
index indexes transaction data in relation to script pubkeys.

This commit also includes initial work on `IndexedTxGraph`.

crates/chain/src/keychain.rs

@@ -19,7 +19,7 @@ use crate::{
     collections::BTreeMap,
     sparse_chain::ChainPosition,
     tx_graph::TxGraph,
-    ForEachTxOut,
+    ForEachTxOut, TxIndexAdditions,
 };
 
 #[cfg(feature = "miniscript")]
@@ -85,6 +85,12 @@ impl<K: Ord> DerivationAdditions<K> {
     }
 }
 
+impl<K: Ord> TxIndexAdditions for DerivationAdditions<K> {
+    fn append_additions(&mut self, other: Self) {
+        self.append(other)
+    }
+}
+
 impl<K> Default for DerivationAdditions<K> {
     fn default() -> Self {
         Self(Default::default())