Make set_hook take a Box<Fn>

Otherwise there's no good way of re-registering a hook you got out of
take_hook.

src/libstd/panic.rs

@@ -29,7 +29,7 @@ pub use panicking::{take_hook, set_hook, PanicInfo, Location};
 #[rustc_deprecated(since = "1.9.0", reason = "renamed to set_hook")]
 #[unstable(feature = "panic_handler", reason = "awaiting feedback", issue = "30449")]
 pub fn set_handler<F>(handler: F) where F: Fn(&PanicInfo) + 'static + Sync + Send {
-    set_hook(handler)
+    set_hook(Box::new(handler))
 }
 
 ///

src/libstd/panicking.rs

@@ -58,12 +58,11 @@ static FIRST_PANIC: AtomicBool = AtomicBool::new(true);
 ///
 /// Panics if called from a panicking thread.
 #[unstable(feature = "panic_handler", reason = "awaiting feedback", issue = "30449")]
-pub fn set_hook<F>(hook: F) where F: Fn(&PanicInfo) + 'static + Sync + Send {
+pub fn set_hook(hook: Box<Fn(&PanicInfo) + 'static + Sync + Send>) {
     if thread::panicking() {
         panic!("cannot modify the panic hook from a panicking thread");
     }
 
-    let hook = Box::new(hook);
     unsafe {
         let lock = HOOK_LOCK.write();
         let old_hook = HOOK;