crypto: add Hashable to VerifiableDecodable

It has to be Hashable.

pkg/crypto/verifiable.go

@@ -1,8 +1,11 @@
 package crypto
 
+import "github.com/nspcc-dev/neo-go/pkg/crypto/hash"
+
 // VerifiableDecodable represents an object which can be verified and
 // those hashable part can be encoded/decoded.
 type VerifiableDecodable interface {
+	hash.Hashable
 	EncodeHashableFields() ([]byte, error)
 	DecodeHashableFields([]byte) error
 }