Use safer casts in Ref<T>

[Zylann]

Ref<T> now uses Object::cast_to, which should be safer. The previous cast could cause undefined behavior if the types were incompatible, now it should return null, which can be checked with .is_valid() or is_null().

I also attempted to add a static_assert to enforce T to derive from Reference, however this would force any mention of Ref<T> to also #include the definition of T, which might not be desirable when forward-declarations are used to speed up compilation (as bindings are currently generated), so I haven't included it.

Should fix #76