feat(chain)!: add take convenience method to Append trait

This is useful if the caller wishes to use the type as a staging area.

This is breaking as `Append` has a `Default` bound now.

crates/wallet/src/wallet/mod.rs

@@ -2291,11 +2291,7 @@ impl Wallet {
 
     /// Take the staged [`ChangeSet`] to be persisted now (if any).
     pub fn take_staged(&mut self) -> Option<ChangeSet> {
-        if self.stage.is_empty() {
-            None
-        } else {
-            Some(core::mem::take(&mut self.stage))
-        }
+        self.stage.take()
     }
 
     /// Get a reference to the inner [`TxGraph`].