[persist_redesign] Introduce redesigned persist types

This is a more generic version of `keychain::persist::*` structures.

Additional changes:

* The `Append` trait has a new method `is_empty`.
* Introduce `Store` structure for `bdk_file_store`.

crates/chain/src/tx_data_traits.rs

@@ -64,20 +64,35 @@ impl<A: Anchor> Anchor for &'static A {
 pub trait Append {
     /// Append another object of the same type onto `self`.
     fn append(&mut self, other: Self);
+
+    /// Returns whether the structure is considered empty.
+    fn is_empty(&self) -> bool;
 }
 
 impl Append for () {
     fn append(&mut self, _other: Self) {}
+
+    fn is_empty(&self) -> bool {
+        true
+    }
 }
 
 impl<K: Ord, V> Append for BTreeMap<K, V> {
     fn append(&mut self, mut other: Self) {
         BTreeMap::append(self, &mut other)
     }
+
+    fn is_empty(&self) -> bool {
+        BTreeMap::is_empty(self)
+    }
 }
 
 impl<T: Ord> Append for BTreeSet<T> {
     fn append(&mut self, mut other: Self) {
         BTreeSet::append(self, &mut other)
     }
+
+    fn is_empty(&self) -> bool {
+        BTreeSet::is_empty(self)
+    }
 }