Return a set from Proof::get_needed_txs()

[St333p]

After writing a test for the Proof::get_needed_txs() we realized that in some cases it returns a list containing duplicated proofs.

fn get_needed_txs(&self) -> Vec<NeededTx> { 

It seems a quick fix with significant performance improvement to make it return a Set.

See src/tests/proof.rs on the "Docs and tests" pull request (#11) for an example, at the third assert there are 2 duplicated txs.