TERRIBLE HACK FIX I DO NOT UNDERSTAND

Okay, so this is the fix for the previously mentioned lifetime problem.  I
mimicked the API for `crossbeam::scope` extremely closely for `Lua::scope`, and
for some reason things that would not compile with `crossbeam::scope` WOULD
compile with `Lua::scope`, and I could not figure it out.

So I took the crossbeam source and made tiny edits until I determined the
crossover point where invalid borrows would compile, and it was.. not what I
expected it to be.  Simply replacing a RefCell<Option<DtorChain<'a>>> with a
PhantomData<&'a ()> would suddenly cause this to compile with crossbeam:

```
struct Test {
    field: i32,
}
crossbeam::scope(|scope| {
    let mut t = Test {
        field: 0,
    };

    scope.spawn(|| t.field = 42);

    drop(t);

    // ...anything
})

```

which is precisely the same problem as `rlua`.

To say I am unsatisfied by this fix is a drastic understatement.  SURELY this
must be a compiler bug?

src/lua.rs

@@ -33,6 +33,7 @@ pub struct Lua {
 pub struct Scope<'scope> {
     lua: &'scope Lua,
     destructors: RefCell<Vec<Box<FnMut(*mut ffi::lua_State) -> Box<Any>>>>,
+    _phantom: PhantomData<RefCell<&'scope ()>>,
 }
 
 // Data associated with the main lua_State via lua_getextraspace.
@@ -326,6 +327,7 @@ impl Lua {
         let mut scope = Scope {
             lua: self,
             destructors: RefCell::new(Vec::new()),
+            _phantom: PhantomData,
         };
         let r = f(&mut scope);
         drop(scope);