Signature of `CoinSelectionAlgorithm::coin_select` has been changed to
take in a `&mut RangCore`. This allows us to pass the random number
generator directly to the cs algorithm.

Single random draw is now it's own type `SingleRandomDraw` and impls
`CoinSelectionAlgorithm`.

`BranchAndBoundCoinSelection` now handles it's own fallback algorithm
internally, and a generic type parameter is added to specify the
fallback algorithm.

`coin_selection::Error` is renamed to `InsufficientFunds` and the BnB
error variants are removed. The BnB error variants are no longer needed
since those cases are handled internally by
`BranchAndBoundCoinSelection` (via calling the fallback algorithm).

Add test_bnb_fallback_algorithm test and docs cleanup suggested by @ValuedMammal.

Added back ignored branch and bounnd tests and cleaned up calculation for target amounts.