Refactors

[Vectorized]

This PR is planned to be merged after #272.

contracts/ERC721A.sol:
    Variable _currentIndex made private.
        Created _nextTokenId() internal view function to return the value instead.
    Variable _burnCounter made private.
        Created _totalBurned() internal view function to return the value instead.

Making these variables private and wrapping them in internal view functions makes them read-only.

This is intended, since devs are NOT supposed to directly edit these values.

It also makes their names more consistent with the functions _totalMinted() and _startTokenId().

Also, this will make it easier to read their values in Diamond upgradeables
(users can simply call _totalBurned() instead of ERC721AUpgradable.storage()._burnCounter).

contracts/extensions/ERC721AOwnersExplicit.sol
    Error QuantityMustBeNonZero renamed to InitializeZeroQuantity for consistency with errors in IERC721A.sol (e.g. MintZeroQuantity).
    Error AllOwnershipsHaveBeenSet renamed to AllOwnershipsInitialized.
    nextOwnerToExplicitlySet renamed to _currentIndexOwnersExplicit and made private for consistency with variables in ERC721A.sol.
        Created _nextTokenIdOwnersExplicit() internal view function to return the value instead.
    Renamed _setOwnersExplicit to _initializeOwnersExplicit to keep consistency with function _initializeOwnershipAt in ERC721A.sol.

Semantically, "initialize" is a better word than "set" for their use case, since users cannot decide on the value.