Merge bitcoindevkit/bdk#1069: Implement Anchor for BlockId

18e8da393779617e8a501d7e21b0a9db1a8b11f1 docs: Add doctest for Anchor implementation on BlockId (Vladimir Fomene)
480c2730de524cbb19a72d8cc8c41cb73ab556df feat: Implement Anchor for BlockId (Vladimir Fomene)

Pull request description:

  ### Description

  This PR fixes #1062.
  It implements the `Anchor` trait on `BlockId`.

  ### 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:
  evanlinjin:
    ACK 18e8da393779617e8a501d7e21b0a9db1a8b11f1

Tree-SHA512: af9da8b4e9c7f2a110340efa4e4c86d9b96ea1275f0a66f08e26a47fcacd89e318a32c845e2a4302f3332eeb4c6a976c3771b1d9881adf4feda90d6ba1a2dd9d