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/writer.go

@@ -0,0 +1,16 @@
+package testutil
+
+import "io"
+
+// ErrWriter returns an io.Writer that returns 0, err from all Write calls.
+func ErrWriter(err error) io.Writer {
+	return &errWriter{err: err}
+}
+
+type errWriter struct {
+	err error
+}
+
+func (r *errWriter) Write(p []byte) (int, error) {
+	return 0, r.err
+}