Merge bitcoindevkit/bdk#1625: feat(chain,core)!: move `Merge` to `bdk_core`

a4cf905d75ad7e09730b6d0aab5ef6cd2e735c2b feat(file_store): rm `bdk_chain` dependency (志宇)
993c4c055354ba6ddb0d81e8e5dbfa1f9bb631ac feat(chain,core)!: move `Merge` to `bdk_core` (志宇)

Pull request description:

  Fixes #1624

  ### Description

  Moving `Merge` into `bdk_core` (from `bdk_chain`) allows persist crates to only depend on `bdk_core`.

  ### Changelog notice

  * Move `Merge` into `bdk_core`.
  * Remove `bdk_chain` dependency from `bdk_file_store`.

  ### 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

ACKs for top commit:
  LagginTimes:
    ACK a4cf905d75ad7e09730b6d0aab5ef6cd2e735c2b
  oleonardolima:
    ACK a4cf905d75ad7e09730b6d0aab5ef6cd2e735c2b
  ValuedMammal:
    ACK a4cf905d75ad7e09730b6d0aab5ef6cd2e735c2b
  notmandatory:
    ACK a4cf905d75ad7e09730b6d0aab5ef6cd2e735c2b

Tree-SHA512: a94e24e04929cf790defd7ca0c8b5194eb5e9b31d295a55f5d3326d3eeb46821eda73191f2f68d8bb7e178bacd2cccfa45c8ded0ce50aabd4b7293edb338c956