Merge bitcoindevkit/bdk#1369: feat(chain): add `get` and `range` methods to `CheckPoint`

53942cced492138174638a8087b08f643a8d41ad chore(chain)!: rm `From<LocalChain> for BTreeMap<u32, BlockHash>` (志宇)
2d1d95a6850a5bab9c5bf369a498fe090852cd93 feat(chain): impl `PartialEq` on `CheckPoint` (志宇)
9a62d56900a33a519dd0165ccdb91711917772f3 feat(chain): add `get` and `range` methods to `CheckPoint` (志宇)

Pull request description:

  Partially fixes #1354

  ### Description

  These methods allow us to query for checkpoints contained within the linked list by height and height range. This is useful to determine checkpoints to fetch for chain sources without having to refer back to the `LocalChain`.

  Currently this is not implemented efficiently, but in the future, we will change `CheckPoint` to use a skip list structure.

  ### Notes to the reviewers

  Please refer to the conversation in #1354 for more details. In summary, both `TxGraph::missing_heights` and `tx_graph::ChangeSet::missing_heights_from` are problematic in their own ways. Additionally, it's a better API for chain sources if we only need to lock our receiving structures twice (once to obtain initial state and once for applying the update). Instead of relying on methods such as `EsploraExt::update_local_chain` to get relevant checkpoints, we can use these query methods instead. This allows up to get rid of `::missing_heights`-esc methods and remove the need to lock receiving structures in the middle of the sync.

  ### Changelog notice

  * Added `get` and `range` methods to `CheckPoint` (and in turn, `LocalChain`). This simulates an API where we have implemented a skip list of checkpoints (to implement in the future). This is a better API because we can query for any height or height range with just a checkpoint tip instead of relying on a separate checkpoint index (which needs to live in `LocalChain`).
  * Changed `LocalChain` to have a faster `Eq` implementation. We now maintain an xor value of all checkpoint block hashes. We compare this xor value to determine whether two chains are equal.
  * Added `PartialEq` implementation for `CheckPoint` and `local_chain::Update`.

  ### 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

ACKs for top commit:
  LLFourn:
    ACK 53942cced492138174638a8087b08f643a8d41ad

Tree-SHA512: 90ef8047fe1265daa54c9dfe8a8c520685c898a50d18efd6e803707fcb529d0790d20373c9e439b9c7ff301db32b47453020cae7db4da2ea64eba895aa047f30