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/rand_js.go

@@ -0,0 +1,21 @@
+package crypto
+
+import (
+	"bufio"
+	"crypto/rand"
+	"io"
+	"sync"
+)
+
+var defaultRand struct {
+	sync.Once
+	reader io.Reader
+}
+
+// DefaultRand returns default entropy source.
+func DefaultRand() io.Reader {
+	defaultRand.Do(func() {
+		defaultRand.reader = bufio.NewReaderSize(rand.Reader, 1024)
+	})
+	return rand.Reader
+}