Mark shadowing variables with weak warning

[zolotov]

Implement shadowing feature of go vet tool (https://golang.org/cmd/vet/#hdr-Shadowed_variables)

https://github.com/golang/tools/blob/master/cmd/vet/shadow.go

[dlsniper]

Maybe we should apply the weak warning to everything, not just variables? Package names can be shadowed for example.

[zolotov]

@dlsniper sounds reasonable

[dlsniper]

Another example of shadowing:

func a() (a, b string, int string, string int) {
    a, b = "1", "2"
    int = "1"
    string = 2
    return
}