go.tools/go/types: add Var.IsField() accessor, and print "field" not "var".
go.tools/go/types: add Var.IsField() accessor, and print "field" not "var".

R=gri
CC=golang-dev
https://codereview.appspot.com/16780043

t-yuki/go.tools.old,t-yuki/go.tools.old,t-yuki/go.tools.old