cmd/vet: use go/printer to pretty-print expressions in printf messages
Fixes issue 4945.
Most examples in this issue now better, but #10 is incomplete and I'm not
certain how to reproduce it. It actually looks like a go/types problem, since
the type being reported is coming directly from that package.
Please reopen the issue if you disagree.

R=golang-dev, rsc
CC=golang-dev
https://codereview.appspot.com/7448046