Move /src to /subprojects

This will facilitate breaking up Nix into multiple packages for each
component with Meson.

subprojects/libutil/unix/os-string.cc

@@ -0,0 +1,21 @@
+#include <algorithm>
+#include <codecvt>
+#include <iostream>
+#include <locale>
+
+#include "file-path.hh"
+#include "util.hh"
+
+namespace nix {
+
+std::string os_string_to_string(PathViewNG::string_view path)
+{
+    return std::string{path};
+}
+
+std::filesystem::path::string_type string_to_os_string(std::string_view s)
+{
+    return std::string{s};
+}
+
+}