Add requested changes

- Change tuple variants for struct variants in `Excess` enum.
- Document `Excess` enum.
- Change parameter `drain_output: &TxOut` by `drain_script: &Script` in
  `coin_selection` method.
- Implement change calculus inside `BranchAndBound::coin_select` algorithms.
- Change `target` by `dust_threshold` to assign `drain_script.dust_value()`.
- Move `change_fee` calculus inside `decide_change`.
- Replace `change_fee` parameter for `fee_rate`.
- Replace FeeRate references by their values

Author: csralvall

CHANGELOG.md

@@ -27,6 +27,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   - Support for `tr()` descriptors in the `descriptor!()` macro
 - Add support for Bitcoin Core 23.0 when using the `rpc` blockchain
 - Add `remove_partial_sigs` and `try_finalize` to `SignOptions`
+- Add `Excess` enum to handle remaining amount after coin selection.
+- Move change creation from `Wallet::create_tx` to `CoinSelectionAlgorithm::coin_select`.
 
 ## [v0.18.0] - [v0.17.0]