Partially revert "Remove the gulrak submodule"

This reverts commit 376518a6ec.

.gitmodules

@@ -1,3 +1,6 @@
 [submodule "ext/gtest"]
 	path = ext/gtest
 	url = https://github.com/google/googletest.git
+[submodule "ext/gulrak-filesystem"]
+	path = ext/gulrak-filesystem
+	url = https://github.com/gulrak/filesystem

src/lib/io/filesystem.cpp

@@ -15,6 +15,9 @@
     along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */
 
+// this header must come first so that it picks up the filesystem implementation
+#include <ghc/fs_impl.hpp>
+
 #include "filesystem.h"
 #if SYSAPI_WIN32
 #include "common/win32/encoding_utilities.h"
@@ -28,7 +31,13 @@ namespace {
 template<class Stream>
 void open_utf8_path_impl(Stream& stream, const fs::path& path, std::ios_base::openmode mode)
 {
+#if SYSAPI_WIN32
+    // on Windows we need to use a non-standard constructor from wchar_t* string
+    // which fs::path::native() returns
     stream.open(path.native().c_str(), mode);
+#else
+    stream.open(path.native().c_str(), mode);
+#endif
 }
 
 } // namespace

src/lib/io/filesystem.h

@@ -20,11 +20,11 @@
 #include <cstdio>
 #include <iosfwd>
 #include <ios>
-#include <filesystem>
+#include <ghc/fs_fwd.hpp>
 
 namespace inputleap {
 
-namespace fs = std::filesystem;
+namespace fs = ghc::filesystem;
 
 void open_utf8_path(std::ifstream& stream, const fs::path& path,
                     std::ios_base::openmode mode = std::ios_base::in);