Merge bitcoindevkit/bdk#694: Add assertions in the FeeRate constructor

235011feef8a6faadc08b814e199e5d5ced2f3a0 Add assertions in the FeeRate constructor (Alekos Filini)

Pull request description:

  ### Description

  Disallow negative, NaN, infinite or subnormal fee rate values.

  ### Notes to the reviewers

  This commit is technically an API break because it makes the `FeeRate::from_sat_per_vb` function non-const. I think it's worth it compared to the risk of having completely nonsensical fee rates (that can break the coin selection in interesting ways).

  EDIT: it's also a breaking change because our code can now panic in scenarios where it didn't before. Again, I think it's worth it.

  ### 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

  #### Bugfixes:

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

ACKs for top commit:
  danielabrozzoni:
    re-ACK 235011feef8a6faadc08b814e199e5d5ced2f3a0

Tree-SHA512: c9432956162fadfd255edf20b825635a487adb29c88d791e18f170da79a2aac6f8e745b5e5be09be3c211697d0b1f4bddc1da75c181e8f9fc4fddf566a7a3e5c