Don't assume first and last comments are aligned

[eugeneius]

Currently we use each_cons(3) to find comments that align with their neighbours, but this means the first and last comments aren't checked. We treat them as always aligned, but this leads to false negatives.

We can instead use each_cons(2) to look at each pair of comments and add both if they're aligned. This does mean that sometimes lines are added twice, but this is fine since Set#<< is idempotent.

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[koic]

Thank you!