This allows `FeeRate`s to be stored inside `const`s.
For example:
const MY_FEE_RATE: FeeRate = FeeRate::from_sat_per_vb(10.0);
Unfortunately, floating point maths inside const expressions is
still unstable, hence we cannot make `from_btc_per_kvb` const.
To allow adding UTXOs external to the current wallet.
The caller must provide the psbt::Input so we can create a coherent PSBT
at the end and so this is compatible with existing PSBT workflows.
Main changes:
- There are now two types of UTXOs, local and foreign reflected in a
`Utxo` enum.
- `WeightedUtxo` now captures floating `(Utxo, usize)` tuples
- `CoinSelectionResult` now has methods on it for distinguishing between
local amount included vs total.
