Clean up pointer use:

* Use unique_ptr for Actions instead of manual memory management
 * Use unique_ptr in KeywordDatabase
 * Use unique_ptr instead of shared_ptr for file formats
 * Don't pass shared_ptr to Reader/Writer, use references instead
Also
 * Switch to C++17 so we can use map::try_emplace

src/data/set.cpp

@@ -223,7 +223,8 @@ void Set::reflect_cards<Writer> (Writer& tag) {
       // writeFile won't quite work because we'd need
       // include file: card: filename
       // to do that
-      Writer writer(openOut(full_name), app_version);
+      auto stream = openOut(full_name);
+      Writer writer(*stream, app_version);
       writer.handle(_("card"), card);
       referenceFile(full_name);
       REFLECT_N("include_file", full_name);