feat: _totalMinted() function.

[Vectorized]

With the burn feature, collections will need to check against _currentIndex if they want to limit the total number of mints.
Checking against totalSupply() can result in new mints being possible if someone burns their token(s).

With upcoming PR #66, users will have to check against _currentIndex - _startTokenId() instead.

This can cause some confusion.

I suggest adding an internal _totalMinted() function,
and update the README to encourage checking against _totalMinted() instead.

Another option is to put this into #66.

Let me know your comments.

If you want to make a PR for this issue, please feel free to do so. Drop a comment below too.

Otherwise, I will probably wait a few / several days after #66 is merged before starting a PR for this (if this feature is not in a PR or merged by then).

@cygaar @ahbanavi

[ahbanavi]

Yes, _totalMinted() method would be convenient for this.
IMO, we better wait for #66 to be merged first.
Putting it in #66 also do the job I think, and it would be faster, but it's out of context of that PR.

[cygaar]

I think I'd prefer it in #66 just so no one accidentally copies this code and runs into an issue checking against _currentIndex. Thoughts on that approach? I'd like to make that change "atomic"

[ahbanavi]

I agree to add in #66. True, being “atomic” is important on this matter.
I will work on this on a fork of #66 and make a PR to @Pczek branch.