Merge bitcoindevkit/bdk#542: Implement XKeyUtils on InnerXKey

b6fe01c4668b1b6737e5e24ae2282f0f55da39cd Implement XKeyUtils on InnerXKey (Gianluca Acerbis)

Pull request description:

  Closes #395

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  ### Description

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  ### Notes to the reviewers

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  ### 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:

  * [ ] I've added tests for the new feature
  * [ ] I've added docs for the new feature
  * [ ] I've updated `CHANGELOG.md`

  #### Bugfixes:

  * [ ] This pull request breaks the existing API
  * [ ] I've added tests to reproduce the issue which are now passing
  * [x] I'm linking the issue being fixed by this PR

ACKs for top commit:
  danielabrozzoni:
    ACK b6fe01c4668b1b6737e5e24ae2282f0f55da39cd - the code looks good to me, I tested locally and all tests pass
  notmandatory:
    ACK b6fe01c4668b1b6737e5e24ae2282f0f55da39cd

Tree-SHA512: 00a3ed9532a0589ac4be55a7d0c6ac5251f03f716adb2086eb934d9a9b04bcb7fd95aaeba38b68c5c90876644ce53ac5e39a912a4096e789887342f8727ae434