Consider changing witness types from hash to btree

[SebastienGllmt]

Currently, in the witness builder we use HashSet. However, we've been told btrees may be easier to use

For instance I generate a tx with the tx builder. Then I take the witness set and merge it with a new one where I only added vkeys. But the order of redeemers and datums could be different now and so it would change the script data hash.
I realised it could make more sense to order Datums and redeemers in the function where I calculate the hash