Add a self-assignment inspection

[dlsniper]

There should be an inspection to determine if we assign a variable to itself:

func a() {
    b := 1
    b = b
   _ = b
}

[dlsniper]

@ignatov I'd like to work on this myself to get used to the way inspections work and this looks quite simple, that's why I didn't assigned it to you. Also it's not really high priority so I've put it into the maybe part as we can do 1.0 without having it :)