Merge bitcoindevkit/bdk#965: Implement persistence with the new structures

4963240599364c6520d0a6ecae97db77cc7b8ba8 Add more `impl`s for `Append` and docs for file store `magic` (志宇)
2aa08a5898545f670df9ed9c4804231f321d811a [persist_redesign] Introduce redesigned `persist` types (志宇)

Pull request description:

  ### Description

  This is part of #895 and #971

  * Introduce a more generic version of the `keychain::persist::*` structures that only needs a single generic for the changeset type.

  Additional changes:

  * The `Append` trait has a new method `is_empty`.
  * Introduce `Store` structure for `bdk_file_store` (which implements `PersistBackend`).

  ### Changelog notice

  ### Checklists

  #### All Submissions:

  * [x] I've signed all my commits
  * [x] I followed the [contribution guidelines](https://github.com/bitcoindevkit/bdk/blob/master/CONTRIBUTING.md)
  * [x] I ran `cargo fmt` and `cargo clippy` before committing

  #### New Features:

  * [x] I've added tests for the new feature
  * [x] I've added docs for the new feature

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Tree-SHA512: 0211fbe7d7e27805d3ed3a80b42f184cdff1cebb32fd559aa9838e4a7f7c7e47b6c366b6ef68e299f876bafed549b8d1d8b8cc0366bf5b61db079504a565b9b4