Move change calculus to coin_select

The former way to compute and create change was inside `create_tx`, just after
performing coin selection.
It blocked the opportunity to have an "ensemble" algorithm to decide between
multiple coin selection algorithms based on a metric, like Waste.

Now, change isn't created inside `coin_select` but the change amount and the
possibility to create change is decided inside the `coin_select` method. In
this way, change is associated with the coin selection algorithm that generated
it, and a method to decide between them can be implemented.

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]