Provide a default CoinSelectionSource implementation via a new trait

Provide a default CoinSelectionSource implementation via a new trait

Certain users may not care how their UTXOs are selected, or their wallet
may not expose enough controls to fully implement the
`CoinSelectionSource` trait. As an alternative, we introduce another
trait `WalletSource` they could opt to implement instead, which is much
simpler as it just returns the set of confirmed UTXOs that may be used.
This trait implementation is then consumed into a wrapper `Wallet` which
implements the `CoinSelectionSource` trait using a "smallest
above-dust-after-spend first" coin selection algorithm.