Making ERC404 _storedERC721Ids internal

[test9955667]

What:
By making _storedERC721Ids in ERC404 internal instead of private, it allows access of the dequeue by 404 extensions without having to edit the core ERC404.sol, and by developers without having to load the state manually. Currently, ERC404 only exposes the length of the dequeue in erc721TokensBankedInQueue().

Why + Example:
An example of an extension that would benefit from this change is a ERC404Viewable.sol. This would expose the queue on the contract api level, lowering the bar for developers who want to access that state. This would make arbitrage of specific 721 ids from the dequeue more accessible for defi power users / devs in general to participate