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/fill_bytes.go

@@ -0,0 +1,15 @@
+package crypto
+
+import "math/big"
+
+// FillBytes is safe version of (*big.Int).FillBytes.
+// Returns false if to length is not exact equal to big.Int's.
+// Otherwise fills to using b and returns true.
+func FillBytes(b *big.Int, to []byte) bool {
+	bits := b.BitLen()
+	if (bits+7)/8 > len(to) {
+		return false
+	}
+	b.FillBytes(to)
+	return true
+}