Switch from string to []byte slices

The way I'm using the base58 required me to do a lot conversions between
[]byte slices and strings. Not anymore.

Also switching to []byte slices allowed some nice performance
improvements.

Before, with strings:

    BenchmarkEncode-4        3000000               511 ns/op
    BenchmarkDecode-4        5000000               300 ns/op

After, with []byte slices:

    BenchmarkEncode-4        5000000               256 ns/op
    BenchmarkDecode-4       20000000               107 ns/op

base58.go

@@ -1,38 +1,41 @@
 package base58
 
 import (
+	"bytes"
 	"errors"
-	"strings"
 )
 
 // Alphabet is the default alphabet.
-const Alphabet = "123456789abcdefghijkmnopqrstuvwxyzABCDEFGHJKLMNPQRSTUVWXYZ"
-const base = len(Alphabet)
+var Alphabet = []byte(
+	"123456789abcdefghijkmnopqrstuvwxyzABCDEFGHJKLMNPQRSTUVWXYZ",
+)
+var base = len(Alphabet)
 
 var errInvalidBase58 = errors.New("invalid base58")
 
 // Encode converts a base10 integer to a base58 string using the default
 // alphabet.
-func Encode(num int) string {
-	str := ""
+func Encode(num int) []byte {
+	str := []byte{}
+
 	for num >= base {
 		mod := num % base
-		str = string(Alphabet[mod]) + str
+		str = prepend(str, Alphabet[mod])
 		num = (num - mod) / base
 	}
 
-	return string(Alphabet[num]) + str
+	return prepend(str, Alphabet[num])
 }
 
 // Decode converts a base58 string to a base10 integer using the default
 // alphabet.
-func Decode(str string) (int, error) {
+func Decode(str []byte) (int, error) {
 	num := 0
 	multi := 1
 
 	for i := len(str); i > 0; i-- {
-		char := string(str[i-1])
-		index := strings.Index(Alphabet, char)
+		char := str[i-1]
+		index := bytes.IndexByte(Alphabet, char)
 		if index == -1 {
 			return -1, errInvalidBase58
 		}
@@ -42,3 +45,10 @@ func Decode(str string) (int, error) {
 
 	return num, nil
 }
+
+func prepend(slice []byte, elem byte) []byte {
+	slice = append(slice, byte(0))
+	copy(slice[1:], slice)
+	slice[0] = elem
+	return slice
+}