feat(persist): introduce CombinedChangeSet

It is a good idea to have common changeset types stored in
`bdk_persist`. This will make it convenient for persistence crates and
`bdk_wallet` interaction.

crates/persist/src/lib.rs

@@ -1,5 +1,8 @@
 #![doc = include_str!("../README.md")]
 #![no_std]
 #![warn(missing_docs)]
+
+mod changeset;
 mod persist;
+pub use changeset::*;
 pub use persist::*;