Merge bitcoindevkit/bdk#630: Move change logic to coin_select

32ae95f463f62c42c6d6aec62c1832a30298fce4 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 -->

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  ### 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 32ae95f463f62c42c6d6aec62c1832a30298fce4

Tree-SHA512: 350adb86538949ff50f41151fc46c8d28d9f5fd659e9869882cc3cb30128d76d4b479512c74c721f8beebfdb5423363ad63368e30556efe65ced2b8c52c34ef6