Properly implement isSigned

[Soupstraw]

At the moment it uses a dummy implementation. We need an implementation that actually checks the signature.

[jmchapman]

• need more info
• a decision how to implement it
• an estimate

[TimSheard]

I think this may be talking about finding witnesses for Keys and Script hashes in a transaction body.
Currently we randomly generate things that look like hashes, but finding the Key or Script that hashes
to that hash is impossible. We will have to randomly generate an universe of pairs (Map Hash KeyPair, Map Hash Script) and then pick hashes from this universe. Then we can figure out how to sign by using this universe.
Of course I could be wrong.

@MaximilianAlgehed and I talked about this today as something we will need to do for conformance testing.