Make fzf pass go vet

Add String() methods to types, so they can be printed with %s. Change
some %s format specifiers to %v, when the default string representation
is good enough. In Go 1.10, `go test` triggers a parallel `go vet`. So
this also makes fzf pass `go test`.

Close #1236
Close #1219

src/tokenizer_test.go

@@ -9,35 +9,35 @@ func TestParseRange(t *testing.T) {
 		i := ".."
 		r, _ := ParseRange(&i)
 		if r.begin != rangeEllipsis || r.end != rangeEllipsis {
-			t.Errorf("%s", r)
+			t.Errorf("%v", r)
 		}
 	}
 	{
 		i := "3.."
 		r, _ := ParseRange(&i)
 		if r.begin != 3 || r.end != rangeEllipsis {
-			t.Errorf("%s", r)
+			t.Errorf("%v", r)
 		}
 	}
 	{
 		i := "3..5"
 		r, _ := ParseRange(&i)
 		if r.begin != 3 || r.end != 5 {
-			t.Errorf("%s", r)
+			t.Errorf("%v", r)
 		}
 	}
 	{
 		i := "-3..-5"
 		r, _ := ParseRange(&i)
 		if r.begin != -3 || r.end != -5 {
-			t.Errorf("%s", r)
+			t.Errorf("%v", r)
 		}
 	}
 	{
 		i := "3"
 		r, _ := ParseRange(&i)
 		if r.begin != 3 || r.end != 3 {
-			t.Errorf("%s", r)
+			t.Errorf("%v", r)
 		}
 	}
 }