Make MerkleNode.collect return a set of MerkleNode instead of a dict

The MerkleNode.collect method enables to return the nodes in a Merkle tree that have been modified or added since the last collect operation.

However it currently returns a dict with the following layout:

{
  'node_type_A': {
    node1.hash: node1.get_data(),
    node2.hash: node2.get_data(),
  },
  'node_type_A': {
    node3.hash: node3.get_data(),
    ...
  },
  ...
}

It would be more convenient if that method simply returns the set of added/modified nodes since the last collect as it would give more flexibility to client code for the processing of those nodes.

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Migrated from T4633 (view on Phabricator)