std: Change Finally to take &mut self

As with the previous commits, the Finally trait is primarily implemented for
closures, so the trait was modified from `&self` to `&mut self`. This will
require that any closure variable invoked with `finally` to be stored in a
mutable slot.

[breaking-change]

src/libstd/unstable/finally.rs

@@ -35,19 +35,19 @@ use ops::Drop;
 #[cfg(test)] use task::failing;
 
 pub trait Finally<T> {
-    fn finally(&self, dtor: ||) -> T;
+    fn finally(&mut self, dtor: ||) -> T;
 }
 
 impl<'a,T> Finally<T> for ||: 'a -> T {
-    fn finally(&self, dtor: ||) -> T {
-        try_finally(&mut (), (),
-                    |_, _| (*self)(),
+    fn finally(&mut self, dtor: ||) -> T {
+        try_finally(&mut (), self,
+                    |_, f| (*f)(),
                     |_| dtor())
     }
 }
 
 impl<T> Finally<T> for fn() -> T {
-    fn finally(&self, dtor: ||) -> T {
+    fn finally(&mut self, dtor: ||) -> T {
         try_finally(&mut (), (),
                     |_, _| (*self)(),
                     |_| dtor())