Merge bitcoindevkit/bdk#718: Verify signatures after signing

7b1ad1b62914a26d6f445364ace4e784bb2901c2 Verify signatures after signing (Scott Robinson)

Pull request description:

  ### Description

  Verify signatures after signing

  As per [BIP-340, footnote 14][fn]:
  > Verifying the signature before leaving the signer prevents random or
  > attacker provoked computation errors. This prevents publishing invalid
  > signatures which may leak information about the secret key. It is
  > recommended, but can be omitted if the computation cost is prohibitive.

  [fn]: https://github.com/bitcoin/bips/blob/master/bip-0340.mediawiki#cite_note-14

  ### Notes to the reviewers

  How do we test this?

  ### Checklists

  #### All Submissions:

  * [ ] 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:
  afilini:
    re-ACK 7b1ad1b62914a26d6f445364ace4e784bb2901c2

Tree-SHA512: 7319db1f8cec2fcfe4ac443ab5728893f9fb6133b33331b35ec6910662c45de8a7cdcf80ac1f3bb435815e914ccf639682a5c07ff0baef42605bf044a34a8232