all: reduce dependencides and be more idiomatic

errors.go

@@ -1,9 +1,5 @@
 package stun
 
-import (
-	"fmt"
-)
-
 // Error is error type for constant errors in stun package.
 //
 // See http://dave.cheney.net/2016/04/07/constant-errors for more info.
@@ -41,18 +37,15 @@ type DecodeErrPlace struct {
 }
 
 func (p DecodeErrPlace) String() string {
-	return fmt.Sprintf("%s/%s", p.Parent, p.Children)
+	return p.Parent + "/" + p.Children
 }
 
 func (e DecodeErr) Error() string {
-	return fmt.Sprintf("BadFormat for %s: %s",
-		e.Place,
-		e.Message,
-	)
+	return "BadFormat for " + e.Place.String() + ": " + e.Message
 }
 
-func newDecodeErr(parent, children, message string) DecodeErr {
-	return DecodeErr{
+func newDecodeErr(parent, children, message string) *DecodeErr {
+	return &DecodeErr{
 		Place:   DecodeErrPlace{Parent: parent, Children: children},
 		Message: message,
 	}