Optimized memory

[pvginkel]

Memory usage has been improved in a lot of places:

• JsFn is now a pointer instead of a closure (saves a word);
• The details of JsFunction has been moved out of JsObject into a native box;
• JsObject::class is not an Option<T> anymore. Instead a magic value is used to mark it as empty;
• Almost all of the JsValue allocations have been removed by creating a JsValue version that embeds a Local<T> (i.e. rooted pointer).