Merge bitcoindevkit/bdk#640: Get balance in categories

0f03831274d3aa69da6e89729c65d66530bbd752 Change get_balance to return in categories. (wszdexdrf)

Pull request description:

  ### Description
  This changes `get_balance()` function so that it returns balance separated in 4 categories:
  - available
  - trusted-pending
  - untrusted-pending
  - immature

  Fixes #238

  ### Notes to the reviewers
  Based on #614

  ### 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:

  * [x] I've updated 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 0f03831274d3aa69da6e89729c65d66530bbd752

Tree-SHA512: 39f02c22c61b6c73dd8e6d27b1775a72e64ab773ee67c0ad00e817e555c52cdf648f482ca8be5fcc2f3d62134c35b720b1e61b311cb6debb3ad651e79c829b93