Add ValueType::is_option

Author: tyt2y3

src/value.rs

@@ -232,6 +232,10 @@ pub trait ValueType: Sized {
         Self::try_from(v).expect(msg)
     }
 
+    fn is_option() -> bool {
+        false
+    }
+
     fn type_name() -> String;
 
     fn array_type() -> ArrayType;
@@ -643,6 +647,10 @@ where
         }
     }
 
+    fn is_option() -> bool {
+        true
+    }
+
     fn type_name() -> String {
         format!("Option<{}>", T::type_name())
     }