Merge bitcoindevkit/bdk#671: Introduce `get_checksum_bytes` method and improvements

6db5b4a094263a07eeb82ba92b47754d25a8d0d3 Introduce `get_checksum_bytes` method and improvements (志宇)

Pull request description:

  ### Description

  `get_checksum_bytes()` returns a descriptor checksum as `[u8; 8]` instead of `String`, potentially improving performance and memory usage.

  In addition to this, since descriptors only use characters that fit within a UTF-8 8-bit code unit ([US-ASCII](https://www.charset.org/charsets/us-ascii)), there is no need to use the `char` type (which is 4 bytes). This can also potentially bring in some performance and memory-usage benefits.

  ### Notes to the reviewers

  This is useful because we will be using descriptor checksums for indexing operations in the near future (multi-descriptor wallets #486 ).

  Refer to comments by @afilini :
  * https://github.com/bitcoindevkit/bdk/pull/647#discussion_r921184366
  * https://github.com/bitcoindevkit/bdk/pull/647#discussion_r921914696
  * https://github.com/bitcoindevkit/bdk/pull/654#discussion_r921980876

  ### 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
  * [x] I've updated `CHANGELOG.md`

ACKs for top commit:
  afilini:
    ACK 6db5b4a094263a07eeb82ba92b47754d25a8d0d3

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