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/clock/system.go

@@ -0,0 +1,50 @@
+package clock
+
+import "time"
+
+type systemTimer struct {
+	timer *time.Timer
+}
+
+func (s systemTimer) C() <-chan time.Time {
+	return s.timer.C
+}
+
+func (s systemTimer) Reset(d time.Duration) {
+	s.timer.Reset(d)
+}
+
+func (s systemTimer) Stop() bool {
+	return s.timer.Stop()
+}
+
+type systemTicker struct {
+	ticker *time.Ticker
+}
+
+func (s systemTicker) C() <-chan time.Time {
+	return s.ticker.C
+}
+
+func (s systemTicker) Stop() {
+	s.ticker.Stop()
+}
+
+func (s systemTicker) Reset(d time.Duration) {
+	s.ticker.Reset(d)
+}
+
+type systemClock struct{}
+
+func (clock systemClock) Ticker(d time.Duration) Ticker {
+	return systemTicker{ticker: time.NewTicker(d)}
+}
+
+func (systemClock) Timer(d time.Duration) Timer {
+	return systemTimer{timer: time.NewTimer(d)}
+}
+
+func (systemClock) Now() time.Time { return time.Now() }
+
+// System Clock.
+var System Clock = systemClock{}