Optimize nested mapping access in ERC721Enumerable

[0xVolosnikov]

Optimized access to nested mapping: avoiding the need to calculate the index of a slot in the storage several times.

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Latest commit: 0edf4db971f2a50a542d6de5b5ec493776a5ee79

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[ernestognw]

I'm not a big fan of this syntax, which I find less readable ... but it saves 76gas on some run.

@ernestognw WDYT?

Also not a fan of the syntax, but I think it's pretty much what we intended here and I'd rather keep the gas savings since I don't see how this might be dangerous or something.