Drop wif.GetVerificationScript(), drop
smartcontract.CreateSignatureRedeemScript(), add GetVerificationScript()
directly to the PublicKey and use it everywhere.
This allows easier reuse of opcodes and in some cases allows to eliminate
dependencies on the whole vm package, like in compiler that only needs opcodes
and doesn't care about VM for any other purpose.
And yes, they're opcodes because an instruction is a whole thing with
operands, that's what context.Next() returns.
