improve abstraction for AssocList, fix bug in optional else (#315)

* improve abstraction for AssocList, fix bug in optional else

* add flag for tracing output, clean up code for output

* turned off tracing by default, updated goldens, removed two examples that use pointers, shouldn't have been there yet

executable_semantics/ast/declaration.cpp

@@ -21,9 +21,8 @@ void StructDeclaration::Print() const {
 void ChoiceDeclaration::Print() const {
   std::cout << "choice " << name << " {" << std::endl;
   for (auto& alternative : alternatives) {
-    std::cout << "alt " << alternative.first << " ";
-    PrintExp(alternative.second);
-    std::cout << ";" << std::endl;
+    std::cout << "alt " << alternative.first << " " << *alternative.second
+              << ";" << std::endl;
   }
   std::cout << "}" << std::endl;
 }