Because we use wrap the file with `BufReader` with the `EntryIter`, we
need to sync the `BufReader`'s position with the file's offset when we
drop the `EntryIter`. Therefore we have a custom drop impl for
`EntryIter`.
* Wrap file reader with `BufReader`. This reduces calls to `read`.
* Wrap file reader with `CountingReader`. This counts the bytes read by
the underlying reader. We can rewind without seeking first.
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`.
