Add unused_enumerated rule

[marcelofabri]

Fixes #619

[codecov-io]

Current coverage is 82.14% (diff: 86.27%)

Merging #998 into master will increase coverage by 0.04%

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[jpsim]

👍. Awesome! Although the comment in #619 still applies, and this will falsely trigger on enumerated() functions other than the ones in the standard library. But I think that'll be rare enough in practice to keep this rule enabled by default.

Feel free to merge once you add the non-triggering example I recommended.

[jpsim]

I think it'd be useful to add a for (idx, _) in bar.enumerated() { }\n non-triggering example.