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/syncio/writer_at_test.go

@@ -0,0 +1,68 @@
+package syncio
+
+import (
+	"bytes"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestBufWriterAt(t *testing.T) {
+	a := require.New(t)
+	f := BufWriterAt{}
+	var err error
+
+	testData := []byte{1, 2, 3, 4, 5}
+	buf := make([]byte, len(testData))
+	offset := len(testData)
+	zeros := bytes.Repeat([]byte{0}, offset)
+
+	_, err = f.WriteAt([]byte{1}, -1)
+	a.Error(err)
+
+	_, err = f.ReadAt([]byte{}, -1)
+	a.Error(err)
+
+	_, err = f.ReadAt([]byte{1}, 0)
+	a.NoError(err)
+
+	// 5 zeros + testdata
+	_, err = f.WriteAt(testData, int64(offset))
+	a.NoError(err)
+	a.Equal(testData, f.buf[offset:])
+	a.Equal(zeros, f.buf[:offset])
+	_, err = f.ReadAt(buf, int64(offset))
+	a.NoError(err)
+	a.Equal(testData, buf)
+
+	// 2 * testdata
+	_, err = f.WriteAt(testData, 0)
+	a.NoError(err)
+	a.Equal(bytes.Repeat(testData, 2), f.buf)
+	_, err = f.ReadAt(buf, 0)
+	a.NoError(err)
+	a.Equal(testData, buf)
+
+	// 3 * testdata
+	_, err = f.WriteAt(testData, int64(offset*2))
+	a.NoError(err)
+	a.Equal(bytes.Repeat(testData, 3), f.buf)
+	_, err = f.ReadAt(buf, int64(offset*2))
+	a.NoError(err)
+	a.Equal(testData, buf)
+
+	// 3 * testdata + 5 zeros + testdata
+	_, err = f.WriteAt(testData, int64(offset*4))
+	a.NoError(err)
+	a.Equal(bytes.Repeat(testData, 3), f.buf[:3*offset])
+	a.Equal(zeros, f.buf[3*offset:4*offset])
+	a.Equal(testData, f.buf[4*offset:])
+	_, err = f.ReadAt(buf, int64(offset*4))
+	a.NoError(err)
+	a.Equal(testData, buf)
+
+	// 5 * testdata
+	_, err = f.WriteAt(testData, int64(offset*3))
+	a.NoError(err)
+	a.Equal(bytes.Repeat(testData, 5), f.buf)
+}