Merkle list and token account update iterator

[mitschabaude]

This PR introduces several provable types to represent dynamic-sized data structures (on which we perform a fixed number of operations)

• MerkleList<T> represents a list which supports push() and pop() (if you pop an empty list, it returns a dummy)
• MerkleListIterator<T> exposes the functionality of popping through the elements of a Merkle list, but without removing them from the list. Main method: next(), which returns a dummy if we're past the end of the list
• AccountUpdateForest is built up from Merkle lists recursively to represent a forest of account updates -- i.e., the children of another account update
  • uses the Merkle list hash that is compatible with how the protocol hashes trees of account updates
  • so, as we iterate through an AccountUpdateForest, we will be given nodes (account updates) along with their child account updates, which are a Merkle list themselves
• TokenAccountUpdateIterator pulls it all together: It iterates deeply through an AccountUpdateForest, stepping down in the tree after every new account update with children is encountered, and stepping up once a layer is done. Since this will be used by token contracts, it supports skipping "inaccessible subtrees", i.e. it skips the children of an update that already doesn't inherit token permissions itself. Assuming that no subtree is skipped, the iteration will return every update in the tree exactly once, in depth-first order. After the full tree has been processed, it returns dummies.
  • the logic is heavily indebted to @mrmr1993's token contract example.
  • there is an exhaustive new unit test for TokenAccountUpdateIterator. It also tests flat iteration through a Merkle list

[mitschabaude]

this is a fixup - Unconstrained was announced in a previous release but not exported

[mitschabaude]

I'm exporting this since I wanted to use this in dogfooding already as well

[Trivo25]

lgtm