Provide single-data generic TL accessor.

Telegram/SourceFiles/codegen/scheme/codegen_scheme.py

@@ -90,6 +90,7 @@ generate({
     'bytes': 'string',
   },
   'builtinInclude': 'mtproto/core_types.h',
+  'optimizeSingleData': True,
 
   'dumpToText': {
     'include': 'mtproto/details/mtproto_dump_to_text.h',

Telegram/SourceFiles/mtproto/core_types.h

@@ -29,6 +29,9 @@ using mtpPingId = uint64;
 using mtpBuffer = QVector<mtpPrime>;
 using mtpTypeId = uint32;
 
+struct NotSingleDataTypePlaceholder {
+};
+
 namespace MTP {
 
 // type DcId represents actual data center id, while in most cases