0xInu~ _packedOwnershipOf optimization, saving 2143 gas on transfers

[0xInuarashi]

Summary:

• mint becomes ~22 gas units cheaper
• transfer of uninitialized tokens (using lookup trace) becomes ~21 gas units more expensive
• transfer of initialized tokens (no lookup trace) becomes ~2143 gas units cheaper

ERC721A: 
1>Mint(a,5): 98279
2>TransferFrom(a,b,3): 107582
3>TransferFrom(a,b,1): 86054
4>TransferFrom(b,a,1): 44343
5>TransferFrom(b,a,3): 39543
6>Mint(a,50): 151064
7>TransferFrom(a,b,50): 200570

ERC721A 0xInu Optimization: 
1>Mint(a,5): 98257
2>TransferFrom(a,b,3): 107603
3>TransferFrom(a,b,1): 86075
4>TransferFrom(b,a,1): 42200
5>TransferFrom(b,a,3): 37400
6>Mint(a,50): 151042
7>TransferFrom(a,b,50): 200591

[0xInuarashi]

The assumption is that, if the packed token data exists, it must already be within-bounds of < _currentIndex
Thus, if the token data exists (and in this case, it is also not burnt), we can save 1 SLOAD by skipping the _currentIndex and go straight to returning the already-initialized token data.

[Vectorized]

Holy shit, this is gigabrain.

[cygaar]

Good catch