Add uninitialized intrinsic

[kengorab]

This effectively just results in a null pointer, but it's cast to be whatever generic value T it's required to be. This can be dangerous of course and requires additional checks, but is useful in cases where performance is important (like with MapEntry<K, V>[] in the Map type); this approach can be leveraged instead of using Option<T>. It's much less ergonomic and type-safe, but it doesn't result in an extra layer of allocation and pointer-indirection.