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/proto/codec/codec.go

@@ -0,0 +1,44 @@
+package codec
+
+import (
+	"encoding/binary"
+	"io"
+
+	"github.com/go-faster/errors"
+
+	"go.mau.fi/mautrix-telegram/pkg/gotd/bin"
+)
+
+// Codec is MTProto transport protocol encoding abstraction.
+type Codec interface {
+	// WriteHeader sends protocol tag if needed.
+	WriteHeader(w io.Writer) error
+	// ReadHeader reads protocol tag if needed.
+	ReadHeader(r io.Reader) error
+	// Write encode to writer message from given buffer.
+	Write(w io.Writer, b *bin.Buffer) error
+	// Read fills buffer with received message.
+	Read(r io.Reader, b *bin.Buffer) error
+}
+
+// TaggedCodec is codec with protocol tag.
+type TaggedCodec interface {
+	Codec
+	// ObfuscatedTag returns protocol tag for obfuscation.
+	ObfuscatedTag() [4]byte
+}
+
+// readLen reads 32-bit integer and validates it as message length.
+func readLen(r io.Reader, b *bin.Buffer) (int, error) {
+	b.ResetN(bin.Word)
+	if _, err := io.ReadFull(r, b.Buf[:bin.Word]); err != nil {
+		return 0, errors.Wrap(err, "read length")
+	}
+	n := int(binary.LittleEndian.Uint32(b.Buf[:bin.Word]))
+
+	if n <= 0 || n > maxMessageSize {
+		return 0, invalidMsgLenErr{n: n}
+	}
+
+	return n, nil
+}