[8.x] Arrayable/collection support for Collection::splice() replacement param

[MarkKremer]

Hi there,

Behavior

The Collection::splice()'s $replacement parameter has different behavior depending on whether a collection or array is passed in. I expected this to be the same. Example:

$data = collect(['foo', 'baz']);
$data->splice(1, 0, ['bar']); // array
dump($data->all());

// Output:
// array:3 [
//   0 => "foo"
//   1 => "bar"
//   2 => "baz"
// ]

$data = collect(['foo', 'baz']);
$data->splice(1, 0, collect(['bar'])); // collection
dump($data->all());

// Output:
// array:3 [
//  0 => "foo"
//  1 => array:1 [
//    0 => "bar"
//  ]
//  2 => "baz"
// ]

This is due to the way PHP's native array_splice method handles objects. Although, I think this is unexpected for the user.

Backwards compatibility

Backwards compatibility is broken for all objects which implements Enumerable, Arrayable, Jsonable, JsonSerializable or Traversable. Other types are cast to an array by the getArrayableItems() method, which is similar to what array_splice does. The latter does not break backward compatibility.