Don't resolve post values

[gballet]

As of #527, pre-values are no longer resolved. I thought it would be easy to do the same thing for post values, but not so: calling IntermediateRoot will both update the tree and hash it, which means that the tree will be collapsed. For this purpose, the proof-passing needs to happen in between the tree finalization and its hashing. This means an interface change.

We could do this right away, but it might be redundant with the MPT witness stuff. I will therefore explore this avenue before implementing it myself. This is an issue to remind myself to revisit this later.