Input selection algorithm

Rebased to vp-withdrawal.

Note I had to make some minor changes to make the test run again.

Also the createInputs and createInputsStore needs to be cleaned up. They are both taking a store right now and especially the createInputsStore name is misleading.
We have votingPoolAddress in withdrawal.go, which is meant to be private and embedded in the public ones (e.g. ChangeAddress). We could embed it here and define SeriesID(), Branch() and Index() methods on it (which seem to be preferred by Conformal over exposing the fields directly), but I think what we want here is to use the WithdrawalAddress concrete type directly, no?