From: Benno Lossin <y86-dev@xxxxxxxxxxxxxx>
Adds the `assume_init` function to `UniqueArc<MaybeUninit<T>>` that
unsafely assumes the value to be initialized and yields a value of type
`UniqueArc<T>`. This function is used when manually initializing the
pointee of an `UniqueArc`.
Signed-off-by: Benno Lossin <y86-dev@xxxxxxxxxxxxxx>