Make `Changeless` metric more useful when combined.

@LLFourn mentioned in a private discussion:

> ... in theory the combined will be looser than each individual one in the sense that it will take more iterations to find the optimal solution for the combined one I think. (A,B) has to find an optimal solution for A but this will not necessarily be optimal for (A,B) so it has to keep looking.

I responded with:

> Would you agree that combining another metric with `Changeless` is only useful if it tightens the bounds? Otherwise, we might as will do `Iterator::filter` on the `BnbIter`.

### Relevant Context

* https://github.com/bitcoindevkit/coin-select/pull/13#discussion_r1452009375