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/testutil/rand.go

@@ -0,0 +1,31 @@
+package testutil
+
+import (
+	"encoding/binary"
+	"math/rand"
+)
+
+// ZeroRand is zero random source.
+type ZeroRand struct{}
+
+// Read implements io.Reader.
+func (ZeroRand) Read(p []byte) (n int, err error) { return len(p), nil }
+
+func randSeed(data []byte) int64 {
+	if len(data) == 0 {
+		return 0
+	}
+
+	seedBuf := make([]byte, 64/8)
+	copy(seedBuf, data)
+
+	return int64(binary.BigEndian.Uint64(seedBuf))
+}
+
+// Rand returns a new rand.Rand with source deterministically initialized
+// from seed byte slice.
+//
+// Zero length seed (or nil) is valid input.
+func Rand(seed []byte) *rand.Rand {
+	return rand.New(rand.NewSource(randSeed(seed))) // #nosec
+}