[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/lib.rs

@@ -33,6 +33,8 @@ pub mod tx_graph;
 pub use tx_data_traits::*;
 mod chain_oracle;
 pub use chain_oracle::*;
+mod persist;
+pub use persist::*;
 
 #[doc(hidden)]
 pub mod example_utils;