hold a reference to the Ruby object

When we have a JS object that proxies a Ruby
object, store a hard reference to the Ruby object
so that it cannot be garbage collected as long as
a reference is being held from within JavaScript.

lib/v8/conversion/class.rb

@@ -40,7 +40,9 @@ class V8::Conversion
         end
 
         def link
-          context.link self[0].Value(), This()
+          external = self[0]
+          This().SetHiddenValue("rr::implementation", external)
+          context.link external.Value(), This()
         end
 
         def construct(cls)