Using FancyByteArray in places.
- FancyByteArray simplifies several call sites and fixes a subtle bug with my signature checking along the way.
--- The bug was that if someone uses an unnecessarily wide opcode like OP_PUSHDATA4 to push a valid 81-byte signature array in the same script, my "remove signature-signing-itself data" code would not detect the two being equal.
--- I should really get around to writing those tests...