base58: move into its own package

It doesn't belong to crypto in any way other than it uses hash function
internally.

pkg/encoding/base58/base58.go

@@ -0,0 +1,46 @@
+package base58
+
+import (
+	"bytes"
+
+	"github.com/CityOfZion/neo-go/pkg/crypto/hash"
+	"github.com/mr-tron/base58"
+	"github.com/pkg/errors"
+)
+
+// CheckDecode implements a base58-encoded string decoding with hash-based
+// checksum check.
+func CheckDecode(s string) (b []byte, err error) {
+	b, err = base58.Decode(s)
+	if err != nil {
+		return nil, err
+	}
+
+	for i := 0; i < len(s); i++ {
+		if s[i] != '1' {
+			break
+		}
+		b = append([]byte{0x00}, b...)
+	}
+
+	if len(b) < 5 {
+		return nil, errors.New("invalid base-58 check string: missing checksum")
+	}
+
+	if !bytes.Equal(hash.Checksum(b[:len(b)-4]), b[len(b)-4:]) {
+		return nil, errors.New("invalid base-58 check string: invalid checksum")
+	}
+
+	// Strip the 4 byte long hash.
+	b = b[:len(b)-4]
+
+	return b, nil
+}
+
+// CheckEncode encodes given byte slice into a base58 string with hash-based
+// checksum appended to it.
+func CheckEncode(b []byte) string {
+	b = append(b, hash.Checksum(b)...)
+
+	return base58.Encode(b)
+}