lib/protocol, vendor: Import luhn code directly

I've changed it incompatibly to fix a correctness bug. Nonetheless, we
should remain incorrect indefinitely.

lib/protocol/luhn_test.go

@@ -0,0 +1,48 @@
+// Copyright (C) 2014 The Protocol Authors.
+
+package protocol
+
+import (
+	"testing"
+)
+
+func TestGenerate(t *testing.T) {
+	// Base 6 Luhn
+	a := luhnAlphabet("abcdef")
+	c, err := a.generate("abcdef")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if c != 'e' {
+		t.Errorf("Incorrect check digit %c != e", c)
+	}
+
+	// Base 10 Luhn
+	a = luhnAlphabet("0123456789")
+	c, err = a.generate("7992739871")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if c != '3' {
+		t.Errorf("Incorrect check digit %c != 3", c)
+	}
+}
+
+func TestInvalidString(t *testing.T) {
+	a := luhnAlphabet("ABC")
+	_, err := a.generate("7992739871")
+	t.Log(err)
+	if err == nil {
+		t.Error("Unexpected nil error")
+	}
+}
+
+func TestValidate(t *testing.T) {
+	a := luhnAlphabet("abcdef")
+	if !a.luhnValidate("abcdefe") {
+		t.Errorf("Incorrect validation response for abcdefe")
+	}
+	if a.luhnValidate("abcdefd") {
+		t.Errorf("Incorrect validation response for abcdefd")
+	}
+}