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/telegram/downloader/sink.go

@@ -0,0 +1,48 @@
+package downloader
+
+import (
+	"context"
+	"io"
+
+	"github.com/go-faster/errors"
+)
+
+func writeAtLoop(w io.WriterAt, toWrite <-chan block) func(context.Context) error {
+	return func(ctx context.Context) error {
+		for {
+			select {
+			case <-ctx.Done():
+				return ctx.Err()
+			case part, ok := <-toWrite:
+				if !ok {
+					return nil
+				}
+
+				_, err := w.WriteAt(part.data, part.offset)
+				if err != nil {
+					return errors.Wrap(err, "write output")
+				}
+			}
+		}
+	}
+}
+
+func writeLoop(w io.Writer, toWrite <-chan block) func(context.Context) error {
+	return func(ctx context.Context) error {
+		for {
+			select {
+			case <-ctx.Done():
+				return ctx.Err()
+			case part, ok := <-toWrite:
+				if !ok {
+					return nil
+				}
+
+				_, err := w.Write(part.data)
+				if err != nil {
+					return errors.Wrap(err, "write output")
+				}
+			}
+		}
+	}
+}