Fine tune Engine size.

src/api/custom_syntax.rs

@@ -402,7 +402,8 @@ impl Engine {
                     parse: Box::new(parse),
                     func: Box::new(func),
                     scope_may_be_changed,
-                },
+                }
+                .into(),
             );
         self
     }

src/api/events.rs

@@ -363,7 +363,7 @@ impl Engine {
             + SendSync
             + 'static,
     ) -> &mut Self {
-        self.debugger_interface = Some(Box::new((Box::new(init), Box::new(callback))));
+        self.debugger_interface = Some((Box::new(init), Box::new(callback)));
         self
     }
 }