Fix use of deprecated Rc:would_unwrap

Author: satoshinm

src/main.rs

@@ -14,7 +14,6 @@
 
 #![recursion_limit="300"]
 #![feature(const_fn)]
-#![feature(rc_would_unwrap)]
 
 extern crate sdl2;
 extern crate zip;

src/ui/mod.rs

@@ -80,7 +80,7 @@ macro_rules! define_elements {
             fn is_unused(&self) -> bool {
                 match *self {
                     $(
-                        Element::$name(ref inner) => Rc::would_unwrap(inner),
+                        Element::$name(ref inner) => Rc::strong_count(inner) == 1,
                     )*
                 }
             }