b79c178ea8 Add fuzz test for PrefilledTransaction (Tobin Harding)
Pull request description:
Add a simple deserialization fuzz test for `PrefilledTransaction`.
Fixes: #460 (I think, I'm not sure if any of the other types need fuzzing?)
ACKs for top commit:
apoelstra:
ACK b79c178ea8
Kixunil:
ACK b79c178ea8
Tree-SHA512: 3e5cbda6b17c36f414bce05b562b5648d82433b185853e6c8aa33ec491b58e527626eb09caca2a988120329efd92dfd3aaad60ef99746d6ac41555160efeb9d6