Switch interpreter Stacks to Ptr (#763)

This also eliminates the ctad wrapper for Stack: I think the leaning is to remove it. It felt worth keeping the constructor because constructing with a single element is a common use-case.

Adds a single-argument constructor for Scope because the `std::list<std::string>()` is common, and eliding it is consistent with what we've done for things like tuples.

I was considering a vector constructor due to the double-Push on line 1139, but thought the Push() semantics may mean that it's better not to provide.

executable_semantics/interpreter/action.cpp

@@ -53,7 +53,7 @@ void Action::Print(llvm::raw_ostream& out) const {
   }
 }
 
-void Action::PrintList(const Stack<Action*>& ls, llvm::raw_ostream& out) {
+void Action::PrintList(const Stack<Ptr<Action>>& ls, llvm::raw_ostream& out) {
   llvm::ListSeparator sep(" :: ");
   for (const auto& action : ls) {
     out << sep << *action;