修改task_t实现，以便于将来支持stop_token。

tutorial/test_async_when_all.cpp

@@ -206,14 +206,14 @@ void test_when_select()
 	{
 		auto dt = rand() % 120;
 		co_await sleep_for(1ms * dt);
-		ch1 << (int)dt;
+		co_await (ch1 << (int)dt);
 	};
 
 	GO
 	{
 		auto dt = rand() % 120;
 		co_await sleep_for(1ms * dt);
-		ch2 << (int)dt;
+		co_await (ch2 << (int)dt);
 	};
 
 	this_scheduler()->run_until_notask();

vs_proj/librf.vcxproj

@@ -131,11 +131,13 @@
       <MultiProcessorCompilation>true</MultiProcessorCompilation>
       <LanguageStandard>stdcpplatest</LanguageStandard>
       <StringPooling>true</StringPooling>
-      <FavorSizeOrSpeed>Size</FavorSizeOrSpeed>
+      <FavorSizeOrSpeed>Speed</FavorSizeOrSpeed>
       <OmitFramePointers>true</OmitFramePointers>
       <InlineFunctionExpansion>AnySuitable</InlineFunctionExpansion>
       <BufferSecurityCheck>false</BufferSecurityCheck>
       <DisableSpecificWarnings>4834;4505</DisableSpecificWarnings>
+      <FunctionLevelLinking>true</FunctionLevelLinking>
+      <EnableParallelCodeGeneration>true</EnableParallelCodeGeneration>
     </ClCompile>
     <Link>
       <SubSystem>Console</SubSystem>
@@ -163,10 +165,10 @@
       <EnablePREfast>true</EnablePREfast>
       <InlineFunctionExpansion>AnySuitable</InlineFunctionExpansion>
       <OmitFramePointers>true</OmitFramePointers>
-      <EnableFiberSafeOptimizations>true</EnableFiberSafeOptimizations>
       <BufferSecurityCheck>false</BufferSecurityCheck>
       <EnableParallelCodeGeneration>true</EnableParallelCodeGeneration>
       <DisableSpecificWarnings>4834;4505</DisableSpecificWarnings>
+      <FunctionLevelLinking>true</FunctionLevelLinking>
     </ClCompile>
     <Link>
       <SubSystem>Console</SubSystem>