Add unused_closure_parameter rule

[marcelofabri]

Fixes #982

This currently doesn't catch some violations:

// 1. If there's a variable with the same name in another object
[1, 2].map { number in
  return otherThing.number
}

// 2. If there're nested closures and the inner one shadows a variable from the other one
[1, 2].map { number in
  return [1, 2].map { number in
    return number + 1
  }
}

1 seems to be possible to be fixed (I just don't have any good ideas). The other one though I'd say is much harder.

Maybe this should be opt-in since it's not a wide-spread convention AFAIK?

[codecov-io]

Current coverage is 82.08% (diff: 95.65%)

Merging #992 into master will increase coverage by 0.15%

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  Lines          6362       6413    +51   
  Methods           0          0          
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+ Hits           5212       5264    +52   
+ Misses         1150       1149     -1   
  Partials          0          0          

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[jpsim]

Do you have a job? Or do you just write awesome SwiftLint rules for a living? 😛

[jpsim]

I made some small changes in d2b9c72, I hope you don't mind. I feel like this could easily be made correctable. Do you think you can do that in this PR, or should we merge this as-is?

[marcelofabri]

Do you have a job? Or do you just write awesome SwiftLint rules for a living? 😛

I'll take that as a compliment 😂

I made some small changes in d2b9c72, I hope you don't mind. I feel like this could easily be made correctable. Do you think you can do that in this PR, or should we merge this as-is?

I think we can merge this now and later make it correctable. I feel we should create a better way to have correctable ASTRules to take advantage of the methods in ASTRule protocol.

[jpsim]

Sounds good! Let's get this in.