Refactor serde impl.

src/serde/serialize.rs

@@ -83,6 +83,7 @@ impl Serialize for Dynamic {
 }
 
 impl Serialize for ImmutableString {
+    #[inline(always)]
     fn serialize<S: Serializer>(&self, ser: S) -> Result<S::Ok, S::Error> {
         ser.serialize_str(self.as_str())
     }