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.

CHANGELOG.md

@@ -5,8 +5,9 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ## [Unreleased]
-
 - Add `descriptor::checksum::get_checksum_bytes` method.
+- Add `Excess` enum to handle remaining amount after coin selection.
+- Move change creation from `Wallet::create_tx` to `CoinSelectionAlgorithm::coin_select`.
 
 ## [v0.20.0] - [v0.19.0]