Implement mruby.value<T> with automated casting.

[dragostis]

[dragostis]

Not really easy to implement.

First of all, this function would require a Box<T: Any>. This would mean changing mrfn! to allocate everything on the heap.

Second of all, there is not easy way of adding Option<T> here. Downcasting Option<_> is not enough, and doing it manually pretty much defeats the purpose.