use resolved to return Fis in stateless internal

[gballet]

Producing verifiable proofs requires to obtain Fis from the tree. The problem with StatelessNode until now, is that they didn't load all the values, and therefore producing these Fis was impossible.

Now that ParseNode loads all the values in unresolved, this becomes possible in some cases, and those cases correspond to the situation in which proofs have to be produced.

This isn't working for stateless leaf nodes, so that PR will not be merged yet.