v0.0.22: Add `uninitialized` intrinsic (#522)

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.