chore: support old chacha20

transport/shadowsocks/shadowstream/old_chacha20.go

@@ -0,0 +1,22 @@
+package shadowstream
+
+import (
+	"crypto/cipher"
+	"github.com/aead/chacha20/chacha"
+)
+
+type chacha20key []byte
+
+func (k chacha20key) IVSize() int {
+	return chacha.NonceSize
+}
+func (k chacha20key) Encrypter(iv []byte) cipher.Stream {
+	c, _ := chacha.NewCipher(iv, k, 20)
+	return c
+}
+func (k chacha20key) Decrypter(iv []byte) cipher.Stream {
+	return k.Encrypter(iv)
+}
+func ChaCha20(key []byte) (Cipher, error) {
+	return chacha20key(key), nil
+}