move gotd fork into repo. (#111)

- update to latest telegram layer
- remove some references to fields in tg.Entities that don't exist in
the schema
- originally added here:
https://github.com/beeper/td/commit/820929062a2ba0104397bc01235ab58a9cff780e
  - referenced here
-
https://github.com/mautrix/telegramgo/commit/124f0967ed195b5a380c9bd02e170ada9710dde3
-
https://github.com/mautrix/telegramgo/commit/4205047aab2e0639217148b5d125bfaab668bd8e

pkg/gotd/crypto/cipher.go

@@ -0,0 +1,24 @@
+package crypto
+
+import "io"
+
+// Cipher is message encryption utility struct.
+type Cipher struct {
+	rand        io.Reader
+	encryptSide Side
+}
+
+// Rand returns random generator.
+func (c Cipher) Rand() io.Reader {
+	return c.rand
+}
+
+// NewClientCipher creates new client-side Cipher.
+func NewClientCipher(rand io.Reader) Cipher {
+	return Cipher{rand: rand, encryptSide: Client}
+}
+
+// NewServerCipher creates new server-side Cipher.
+func NewServerCipher(rand io.Reader) Cipher {
+	return Cipher{rand: rand, encryptSide: Server}
+}