Replace next(node.infer()) calls with iteration over the all results of node.infer()

[pylint-bot]

Originally reported by: Claudiu Popa (BitBucket: PCManticore, GitHub: @PCManticore)

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node.infer() returns a generator of inferred statements, but there are a lot of places where only the first value is retrieved and operated with. This is wrong, because it doesn't reflect the whole reality of the inference.

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• Bitbucket: https://bitbucket.org/logilab/astroid/issue/136