Add aux to AddressData

[Vectorized]

After adding the burn feature, the AddressData struct still has 64 bits of unused space.

This is free real estate.

This PR adds an aux variable for the AddressData to let users make full use of the extra space,
along with the relevant _getAux and _setAux functions.

This aux can be used for storing miscellaneous things like number of whitelist mint slots used.

Deriving contracts can avoid the overhead of an additional expensive SLOAD and SSTORE with this feature.

If there are no likely plans to use the extra 64 bits of space for future features, we can consider this.

[ahbanavi]

Looks good and yes it's the perfect place for storing whitelist mints.

[cygaar]

Can you add a couple unit tests to verify the behavior? Should be simple since you're just checking that the setter/getter works.

[cygaar]

@Vectorized for some reason you have changes from the past 2 merged PRs in your PR. It might be worth rebasing your code on top of main instead of trying to do a merge commit. Not too sure what happened. Other than that, the PR looks good