smartcontract: add JSON marshal/unmarshal for InteropType

We actually have to do that in order to answer getapplicationlog requests for
transactions that leave some interop items on the stack. It follows the same
logic our binary serializer/deserializes does leaving the type and stripping
the value (whatever that is).

pkg/smartcontract/parameter.go

@@ -85,6 +85,8 @@ func (p *Parameter) MarshalJSON() ([]byte, error) {
 	case MapType:
 		ppair := p.Value.([]ParameterPair)
 		resultRawValue, resultErr = json.Marshal(ppair)
+	case InteropInterfaceType:
+		resultRawValue = []byte("null")
 	default:
 		resultErr = errors.Errorf("Marshaller for type %s not implemented", p.Type)
 	}
@@ -166,6 +168,9 @@ func (p *Parameter) UnmarshalJSON(data []byte) (err error) {
 			return
 		}
 		p.Value = h
+	case InteropInterfaceType:
+		// stub, ignore value, it can only be null
+		p.Value = nil
 	default:
 		return errors.Errorf("Unmarshaller for type %s not implemented", p.Type)
 	}