stream: add file::exists and file::readonly

We are targeting C++14, while C++17 already has std::filesystem::exists. 😢

Signed-off-by: Simon Rozman <simon@rozman.si>

UnitTests/stream.cpp

@@ -139,6 +139,13 @@ namespace UnitTests
 				std::filesystem::remove(filename[i]);
 		}
 
+		TEST_METHOD(file_stat)
+		{
+			sstring path(temp_path());
+			Assert::IsTrue(stdex::stream::file::exists(path));
+			Assert::IsFalse(stdex::stream::file::readonly(path));
+		}
+
 	protected:
 		static sstring temp_path()
 		{