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/salt.go

@@ -0,0 +1,17 @@
+package crypto
+
+import (
+	"encoding/binary"
+
+	"github.com/go-faster/xor"
+
+	"go.mau.fi/mautrix-telegram/pkg/gotd/bin"
+)
+
+// ServerSalt computes server salt.
+func ServerSalt(newNonce bin.Int256, serverNonce bin.Int128) (salt int64) {
+	var serverSalt [8]byte
+	copy(serverSalt[:], newNonce[:8])
+	xor.Bytes(serverSalt[:], serverSalt[:], serverNonce[:8])
+	return int64(binary.LittleEndian.Uint64(serverSalt[:]))
+}