[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/file_store/src/lib.rs

@@ -1,10 +1,51 @@
 #![doc = include_str!("../README.md")]
-mod file_store;
+mod entry_iter;
+mod keychain_store;
+mod store;
+use std::io;
+
 use bdk_chain::{
     keychain::{KeychainChangeSet, KeychainTracker, PersistBackend},
     sparse_chain::ChainPosition,
 };
-pub use file_store::*;
+use bincode::{DefaultOptions, Options};
+pub use entry_iter::*;
+pub use keychain_store::*;
+pub use store::*;
+
+pub(crate) fn bincode_options() -> impl bincode::Options {
+    DefaultOptions::new().with_varint_encoding()
+}
+
+/// Error that occurs due to problems encountered with the file.
+#[derive(Debug)]
+pub enum FileError<'a> {
+    /// IO error, this may mean that the file is too short.
+    Io(io::Error),
+    /// Magic bytes do not match what is expected.
+    InvalidMagicBytes { got: Vec<u8>, expected: &'a [u8] },
+}
+
+impl<'a> core::fmt::Display for FileError<'a> {
+    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+        match self {
+            Self::Io(e) => write!(f, "io error trying to read file: {}", e),
+            Self::InvalidMagicBytes { got, expected } => write!(
+                f,
+                "file has invalid magic bytes: expected={:?} got={:?}",
+                expected, got,
+            ),
+        }
+    }
+}
+
+impl<'a> From<io::Error> for FileError<'a> {
+    fn from(value: io::Error) -> Self {
+        Self::Io(value)
+    }
+}
+
+impl<'a> std::error::Error for FileError<'a> {}
 
 impl<K, P> PersistBackend<K, P> for KeychainStore<K, P>
 where