Add requested changes

- Change tuple variants for struct variants in `Excess` enum.
- Document `Excess` enum.
- Change parameter `drain_output: &TxOut` by `drain_script: &Script` in
  `coin_selection` method.
- Implement change calculus inside `BranchAndBound::coin_select` algorithms.
- Change `target` by `dust_threshold` to assign `drain_script.dust_value()`.

Author: csralvall