lib/protocol, vendor: Import luhn code directly

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

vendor/github.com/calmh/luhn/LICENSE

@@ -1,19 +0,0 @@
-Copyright (C) 2014 Jakob Borg
-
-Permission is hereby granted, free of charge, to any person obtaining a copy of
-this software and associated documentation files (the "Software"), to deal in
-the Software without restriction, including without limitation the rights to
-use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies
-of the Software, and to permit persons to whom the Software is furnished to do
-so, subject to the following conditions:
-
-- The above copyright notice and this permission notice shall be included in
-  all copies or substantial portions of the Software.
-
-THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
-IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
-FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
-AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
-LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
-OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
-SOFTWARE.

vendor/github.com/calmh/luhn/luhn.go

@@ -1,67 +0,0 @@
-// Copyright (C) 2014 Jakob Borg
-
-// Package luhn generates and validates Luhn mod N check digits.
-package luhn
-
-import (
-	"fmt"
-	"strings"
-)
-
-// An alphabet is a string of N characters, representing the digits of a given
-// base N.
-type Alphabet string
-
-var (
-	Base32 Alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZ234567"
-)
-
-// Generate returns a check digit for the string s, which should be composed
-// of characters from the Alphabet a.
-func (a Alphabet) Generate(s string) (rune, error) {
-	factor := 1
-	sum := 0
-	n := len(a)
-
-	for i := range s {
-		codepoint := strings.IndexByte(string(a), s[i])
-		if codepoint == -1 {
-			return 0, fmt.Errorf("Digit %q not valid in alphabet %q", s[i], a)
-		}
-		addend := factor * codepoint
-		if factor == 2 {
-			factor = 1
-		} else {
-			factor = 2
-		}
-		addend = (addend / n) + (addend % n)
-		sum += addend
-	}
-	remainder := sum % n
-	checkCodepoint := (n - remainder) % n
-	return rune(a[checkCodepoint]), nil
-}
-
-// Validate returns true if the last character of the string s is correct, for
-// a string s composed of characters in the alphabet a.
-func (a Alphabet) Validate(s string) bool {
-	t := s[:len(s)-1]
-	c, err := a.Generate(t)
-	if err != nil {
-		return false
-	}
-	return rune(s[len(s)-1]) == c
-}
-
-// NewAlphabet converts the given string an an Alphabet, verifying that it
-// is correct.
-func NewAlphabet(s string) (Alphabet, error) {
-	cm := make(map[byte]bool, len(s))
-	for i := range s {
-		if cm[s[i]] {
-			return "", fmt.Errorf("Digit %q non-unique in alphabet %q", s[i], s)
-		}
-		cm[s[i]] = true
-	}
-	return Alphabet(s), nil
-}