Merge bitcoindevkit/bdk#505: Using dust value from rust-bitcoin in ``is_dust``

5ac51dfe74c51c83fe270570d7ffc50888cf554f fix and test is_dust (James Taylor)
a0c140bb29ab37103e4a24d296ac6fcea91da69e add doc comment for IsDust trait (James Taylor)
bf5994b14ade1fc0c37d3e3cda6946c26ceea631 fixed fee in test, removed unnecessary comment (James Taylor)
ca682819b396c44c9139086b1537d32b212499a1 using dust value from rust-bitcoin (James Taylor)

Pull request description:

  ### Description

  This PR aims to fix #472 . We can retrieve the dust value for a given ``bitcoin::blockdata::script::Script``, so I adjusted the ``is_dust`` function within the ``IsDust`` trait to receive such a ``&Script``. Thus, the ``is_dust`` function can make the proper comparison.
  Let me know if you think that there could be a better interface than this.

  Furthermore, because this new ``is_dust`` function provides a tighter upper bound on Bitcoin Core's ``GetDustThreshold()``, it actually invalidated a test. In the test, the drain output for a transaction was no longer considered dust and no longer included in the fee. Instead, the drain output was kicked back to the sender, invalidating the asserts in line 3436, 3437 and 3441 in ``src/wallet/mod.rs``. I increased the ``FeeRate`` in the test just enough that the drain output would be small enough to considered dust again and included in the total fee.

  ### 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

ACKs for top commit:
  afilini:
    ACK 5ac51dfe74c51c83fe270570d7ffc50888cf554f
  notmandatory:
    re ACK 5ac51dfe74c51c83fe270570d7ffc50888cf554f

Tree-SHA512: addf38fe065de581ddfcd3b4e6db92cd35d5bfa8cac78bd08c01f7a01292724a203ef59b09f3f5cd8e0fa0bb6d89efe72afda36efc11ded0424fc8105326af3f