Merge bitcoindevkit/bdk#2115: Add `prev_blockhash` validation to `CheckPoint`

Merge bitcoindevkit/bdk#2115: Add `prev_blockhash` validation to `CheckPoint`

c486ba75865806bbe1d88c646385ac57df611ef5 docs: address review feedback on `prev_blockhash` validation (志宇)
031b30fb3fbc19894c85dccc0d6a5515442e5bce docs(core): address review feedback on docs and tests (志宇)
ef35b199376b3b88bc7b8985aa1afa6c81bda5c1 fix(chain)!: make genesis immutable in `merge_chains` (志宇)
d8cb41f2c22ed684fe493b29c86de02ddc921423 test(core): address review feedback on checkpoint tests (志宇)
9c0453c7d9767002b6722c888ac1f694c792c0d2 docs(core): Add module-level docs for `checkpoint_entry` (志宇)
3ef6ed866d364d6f8c9136d0b6696f2b90e572be feat(chain)!: Add `ApplyBlockError` for `prev_blockhash` validation (志宇)
bf6541b43d3d65f20a7c081129197337f09d25b6 feat(chain)!: Relax the generic parameter for `LocalChain<D>` (志宇)
19b900648f9308d671849c927abf4084d6843942 test(core): add tests for CheckPoint::push and insert methods (志宇)
9971fda5367cfa69917445a11c4b8c9d6de3f713 test(chain): Test `apply_update` with a single `CheckPoint<Header>` (valued mammal)
3df06096f9209a0ed12e109e0d809174a7b1a73b test(chain): make `TestLocalChain` generic and add `prev_blockhash` test (志宇)
c314b2567b2574da38e060bb57c02c9929082c08 fix(chain): `merge_chains` now takes account of `prev_blockhash`es (志宇)
f10ba0ecb1e60aef40820298a357eb394bc190e9 fix(core): `Checkpoint::insert` now evicts on `prev_blockhash` mismatch (志宇)
046a771b7842dac74d5cb82a56b28fbbbbe1a33a fix(core): `push` now errors on `prev_blockhash` mismatch (志宇)
68d1ef4466cf64104b0338a3feed902080a35e05 feat(core): Initial work on `CheckPointEntry` (志宇)
d4bdff08f7de754461d9364302b7e8371e3d234e feat(core): Add `prev_blockhash` method to `ToBlockHash` trait (志宇)

Pull request description:

  Closes #2021
  Related to #2076
  Replaces #2024
  Replaces #2091

  ### Description

  This PR adds `prev_blockhash` awareness to `CheckPoint`, enabling proper chain validation when merging checkpoint chains that store block headers or similar data with previous block hash information.

  ### Notes to the reviewers

  This PR replaces some prior attempts:

  * #2024 - where we made the `CheckPoint::data` optional - however this resulted in internal complexity and an API with annoying edge cases. The tests from this PR were still useful.

  * #2091 - This second attempt had some good ideas, but was distracted from the goal of #2021. I mostly reused the `CheckPoint::insert` implementation of that PR.

  ### Changelog notice

  ```md
  Added:
  - `ToBlockHash::prev_blockhash()` - optional method to expose previous block hash
  - `CheckPointEntry` - new type for iterating with `prev_blockhash` awareness, yielding "placeholder" entries for heights inferred from `prev_blockhash`
  - `ApplyBlockError` - this is a new error type with two variants; `MissingGenesis` and `PrevBlockhashMismatch`. The second variant is a new error case introduced by `prev_blockhash` awareness.

  Changed:
  - `CheckPoint::push` - now errors when `prev_blockhash` conflicts with current tip (contiguous heights)
  - `CheckPoint::insert` - now evicts/displaces checkpoints on `prev_blockhash` conflict
  - `merge_chains` - now validates `prev_blockhash` consistency when merging
  - `LocalChain<D>` generic parameter - relaxed constraint to `D: Clone` instead of `D: Copy`.

  Fixed:
  - `merge_chains` no longer replaces the genesis block.
  ```

  ### Checklists

  #### All Submissions:

  * [x] I followed the [contribution guidelines](https://github.com/bitcoindevkit/bdk/blob/master/CONTRIBUTING.md)

  #### New Features:

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

ACKs for top commit:
  evanlinjin:
    self-ACK c486ba75865806bbe1d88c646385ac57df611ef5

Tree-SHA512: 5622a8a418034443931c8b5938e708c2222c42c05efbdac71d47a4e0ce1b4f4b0b7bcd2e1e14e2f295cc056e12c03e5750b40b1a1313cdbdf59a009d81d0b8a1