Remember that we will be moving Update into a separate crate bdk_core so we won't have access to private members.

Additionally, we don't want to provide floating outputs to our update logic when we already have the full tx in the same update struct.

With these two requirements in mind, let's see various solutions.

• Publicize all members. This is a bad idea, since we can't enforce our invariances.
• Enforce invariance when caller inserts data. When inserting full tx, rm all associated floating txouts of the tx's txid. Don't insert floating txout when there is a full tx of same txid.
• Enforce invariance when extracting data. When extracting full txs, also rm floating txouts with the same txid. When extracting floating txouts, ignore elements which have an associated full tx.

I think enforcing invariance when caller inserts data is easier to reason with.