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/check_gp_test.go

@@ -0,0 +1,55 @@
+package crypto
+
+import (
+	"encoding/hex"
+	"math/big"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+// From Telegram docs example.
+//
+// See https://core.telegram.org/mtproto/auth_key#presenting-proof-of-work-server-authentication.
+var (
+	checkGPdhPrime = func() *big.Int {
+		data, err := hex.DecodeString("C71CAEB9C6B1C9048E6C522F70F13F73980D40238E3E21C14934D037563D930F" +
+			"48198A0AA7C14058229493D22530F4DBFA336F6E0AC925139543AED44CCE7C37" +
+			"20FD51F69458705AC68CD4FE6B6B13ABDC9746512969328454F18FAF8C595F64" +
+			"2477FE96BB2A941D5BCD1D4AC8CC49880708FA9B378E3C4F3A9060BEE67CF9A4" +
+			"A4A695811051907E162753B56B0F6B410DBA74D8A84B2A14B3144E0EF1284754" +
+			"FD17ED950D5965B4B9DD46582DB1178D169C6BC465B0D6FF9CA3928FEF5B9AE4" +
+			"E418FC15E83EBEA0F87FA9FF5EED70050DED2849F47BF959D956850CE929851F" +
+			"0D8115F635B105EE2E4E15D04B2454BF6F4FADF034B10403119CD8E3B92FCC5B")
+		if err != nil {
+			panic(err)
+		}
+
+		return big.NewInt(0).SetBytes(data)
+	}()
+	checkGPg = 3
+)
+
+func BenchmarkCheckGP(b *testing.B) {
+	b.ReportAllocs()
+	b.ResetTimer()
+
+	for i := 0; i < b.N; i++ {
+		_ = CheckGP(checkGPg, checkGPdhPrime)
+	}
+}
+
+func TestCheckGP(t *testing.T) {
+	t.Run("OK", func(t *testing.T) {
+		a := require.New(t)
+		a.NoError(CheckGP(checkGPg, checkGPdhPrime))
+	})
+	t.Run("WrongG", func(t *testing.T) {
+		require.Error(t, CheckGP(1337, checkGPdhPrime))
+	})
+	t.Run("WrongDivider", func(t *testing.T) {
+		// CheckGP should check that p mod 3 = 2 for g = 3;
+		// We pass p = 4, so p mod 3 = 1.
+		require.Error(t, CheckGP(3, big.NewInt(4)))
+	})
+}