Implement hash preimage verifier

[karlfloersch]

Create a super simple hash preimage verifier which will be used by the witness exists predicate. This should resemble:

PreimageVerifierParameters = {
    hashImage: bytes32
}

contract IS_PREIMAGE_VERIFIER {
    verify(_parameters: PreimageVerifierParameters, _witness: bytes): bool {
        return (hash(_witness) == _parameters.hashImage)
    }
}