remove GC histrionics in backreferences.

The memory address for the storage cell does not
change, only the VALUE, so there is no need to 
unregister and register the storage cell pointer
with Ruby GC every time its value changes.

ext/v8/backref.cc

@@ -14,32 +14,23 @@ namespace rr {
   }
 
   Backref::Backref(VALUE initial) {
-    allocate(initial);
+    set(initial);
+    rb_gc_register_address(&storage);
   }
 
   Backref::~Backref() {
-    deallocate();
+    rb_gc_unregister_address(&storage);
   }
 
-  void Backref::allocate(VALUE data) {
+  VALUE Backref::set(VALUE data) {
     this->storage = rb_funcall(Storage, _new, 1, data);
-    rb_gc_register_address(&storage);
-  }
-
-  void Backref::deallocate() {
-    rb_gc_unregister_address(&storage);
+    return data;
   }
 
   VALUE Backref::get() {
     return rb_funcall(storage, object, 0);
   }
 
-  VALUE Backref::set(VALUE data) {
-    deallocate();
-    allocate(data);
-    return data;
-  }
-
   v8::Handle<v8::Value> Backref::toExternal() {
     v8::Local<v8::Value> wrapper = v8::External::Wrap(this);
     v8::Persistent<v8::Value>::New(wrapper).MakeWeak(this, &release);

ext/v8/rr.h

@@ -268,8 +268,6 @@ class Backref {
   v8::Handle<v8::Value> toExternal();
   static void release(v8::Persistent<v8::Value> handle, void* data);
 private:
-  void allocate(VALUE data);
-  void deallocate();
   VALUE storage;
   static VALUE Storage;
   static ID _new;