vs2019 and std::filesystem

[baconpaul]

vs2019 seems to have put std::filesystem in std:: rather than std::experimental

I think this

+#if WINDOWS && ( _MSC_VER >= 1920 )
+// vs2019
+namespace fs = std::filesystem
+#else
 namespace fs = std::experimental::filesystem;
+#endif

is the type of diff we will need. But I want to install vs2019 and try. And it may not work anyway since we force build system to 141 (vs2017).

Tagging @VincyZed