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/field/choice.cpp

@@ -190,7 +190,7 @@ void ChoiceStyle::initImage() {
   //       CALL       0
   //       PUSH_CONST nil
   //      OR_ELSE
-  ScriptCustomCollectionP lookup(new ScriptCustomCollection());
+  ScriptCustomCollectionP lookup = make_intrusive<ScriptCustomCollection>();
   FOR_EACH(ci, choice_images) {
     lookup->key_value[uncanonical_name_form(ci.first)] = 
       lookup->key_value[ci.first] = ci.second.getValidScriptP();