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/srp/pad.go

@@ -0,0 +1,23 @@
+package srp
+
+import (
+	"math/big"
+
+	"go.mau.fi/mautrix-telegram/pkg/gotd/crypto"
+)
+
+func (s SRP) pad256FromBig(i *big.Int) (b [256]byte, r bool) {
+	r = crypto.FillBytes(i, b[:])
+	return b, r
+}
+
+func (s SRP) pad256(b []byte) [256]byte {
+	if len(b) >= 256 {
+		return *(*[256]byte)(b[len(b)-256:])
+	}
+
+	var tmp [256]byte
+	copy(tmp[256-len(b):], b)
+
+	return tmp
+}