Added a sanity check to the CoinSpend class
This check verifies that the Witness is correct w.r.t. the given Accumulator
and PublicCoin /before/ constructing the zero knowledge proofs.
This required an API change to the AccumulatorWitness.Verify() method
to allow caller to provide an explicit PublicCoin to check.
Also modified Tests.cpp to support new AccumulatorWitness.Verify() API.