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/prime.go

@@ -0,0 +1,16 @@
+package crypto
+
+import "math/big"
+
+// Prime checks that given number is prime.
+func Prime(p *big.Int) bool {
+	// TODO(tdakkota): maybe it should be smaller?
+	// 1 - 1/4^64 is equal to 0.9999999999999999999999999999999999999970612641229442812300781587
+	//
+	// TDLib uses nchecks = 64
+	// See https://github.com/tdlib/td/blob/d161323858a782bc500d188b9ae916982526c262/tdutils/td/utils/BigNum.cpp#L155.
+	const probabilityN = 64
+
+	// ProbablyPrime is mutating, so we need a copy
+	return p.ProbablyPrime(probabilityN)
+}