Array Initializer with Access to Uninitialized Buffer

[benrimmington]

@nnnnnnnn

For Array, we would want to guarantee that the array has storage for exactly the specified number of elements.

But the Array.reserveCapacity(_:) API doesn't guarantee an exact capacity either:

  /// For performance reasons, the size of the newly allocated storage might be
  /// greater than the requested capacity. Use the array's `capacity` property
  /// to determine the size of the new storage.