Merge #630: Move change logic to coin_select

32ae95f Move change calculus to coin_select (Cesar Alvarez Vallero)

Pull request description:

  ### Description

  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 is not 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.

  Fixes #147.
  <!-- Describe the purpose of this PR, what's being adding and/or fixed -->

  <!-- In this section you can include notes directed to the reviewers, like explaining why some parts
  of the PR were done in a specific way -->

  ### 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

  #### New Features:

  * [ ] I've added tests for the new feature
  * [x] I've added docs for the new feature
  * [x] I've updated `CHANGELOG.md`

ACKs for top commit:
  afilini:
    ACK 32ae95f

Tree-SHA512: 350adb86538949ff50f41151fc46c8d28d9f5fd659e9869882cc3cb30128d76d4b479512c74c721f8beebfdb5423363ad63368e30556efe65ced2b8c52c34ef6

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]