Rewrite vfs::get and vfs::mount

Preprocess . and .. correctly
Don't use recursive locking
Also use std::string_view
Fix format system for std::string and std::string_view
Fix fmt::merge for std::string_view

rpcs3/stdafx.h

@@ -43,5 +43,6 @@ namespace std { inline namespace literals { inline namespace chrono_literals {}}
 #include <functional>
 #include <unordered_map>
 #include <algorithm>
+#include <string_view>
 
 using namespace std::literals;