cmd/vet: printf findings can be arbitrarily long

[adonovan]

When vet reports a printf problem such as this one:

fmt.Printf("%d", func() { ... }) // arg in printf call is a function value, not a function call

it prints out the entire body of the function literal, which can be quite long.
For example, given the program at https://play.golang.org/p/f8W0zpISww, vet prints

% go vet ~/a.go
../../../../a.go:6: arg func() {
        println("blah")
        println("blah")
        println("blah")
... dozens more lines...
        println("blah")
        println("blah")
        println("blah")
} in printf call is a function value, not a function call

Perhaps it should not print the function body at all.

[josharian]

cc @valyala

[valyala]

Currently long vet output isn't limited by -printf check and/or by long function bodies. For instance, complex composite literal may result in a long vet output too.