chore(chain): Standardise KeychainTxOutIndex return types

The previous commit b9c5b9d08b040faf6c6b2d9b3745918031555b72 added
IndexSpk. This goes further and adds `Indexed` and `KeychainIndexed`
type alises (IndexSpk is Indexed<ScriptBuf>) and attempts to standardize
the structure of return types more generally.

crates/chain/src/keychain.rs

@@ -12,7 +12,7 @@
 
 #[cfg(feature = "miniscript")]
 mod txout_index;
-use bitcoin::Amount;
+use bitcoin::{Amount, ScriptBuf};
 #[cfg(feature = "miniscript")]
 pub use txout_index::*;
 
@@ -49,6 +49,11 @@ impl Balance {
     }
 }
 
+/// A tuple of keychain index and corresponding [`ScriptBuf`].
+pub type Indexed<T> = (u32, T);
+/// A tuple of keychain, index and the [`ScriptBuf`] derived at that location.
+pub type KeychainIndexed<K, T> = ((K, u32), T);
+
 impl core::fmt::Display for Balance {
     fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
         write!(