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.

Author: csralvall

CHANGELOG.md

@@ -10,6 +10,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Fix hang when `ElectrumBlockchainConfig::stop_gap` is zero.
 - Set coin type in BIP44, BIP49, and BIP84 templates
 - Get block hash given a block height - A `get_block_hash` method is now defined on the `GetBlockHash` trait and implemented on every blockchain backend. This method expects a block height and returns the corresponding block hash. 
+- Add `Excess` enum to handle remaining amount after coin selection.
+- Move change creation from `Wallet::create_tx` to `CoinSelectionAlgorithm::coin_select`.
 
 ## [v0.19.0] - [v0.18.0]