Remove Rc wrapping of BuiltinFnPtr in BuiltinFn wrapper

I'm trying to remember why this was added in the first place - I think
to simplifly cloning? It's not important now though.

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

src/vm/machine.rs

@@ -332,7 +332,7 @@ impl MachineBuilder {
     ) {
         self.register_global(
             name,
-            BuiltinFnObj::new(BuiltinFn::new(name.to_string(), Rc::new(fun))).into_gc(),
+            BuiltinFnObj::new(BuiltinFn::new(name.to_string(), fun)).into_gc(),
         );
     }