Change UtxoSet to use full TXID as key

Sorry, this is needed to enable proper txid/vout lookups for the address index.
This means any users of wizards-wallet need to rebuild their utxo sets, and
will also mean an increase in RAM usage.
Fix for upstream
Use `verify_raw` rather than `verify` to check signatures

`verify` cannot handle illegally padded signatures because it takes an object
of type `Signature`, which is a fixed-size type. This should have been part
of the previous commit --- an important lesson about running the unit tests
before every push!