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/interpreter/value.cpp

@@ -314,6 +314,11 @@ void PrintValue(Value* val, std::ostream& out) {
   }
 }
 
+auto operator<<(std::ostream& out, const Value& v) -> std::ostream& {
+  PrintValue((Value*)&v, out);
+  return out;
+}
+
 auto TypeEqual(Value* t1, Value* t2) -> bool {
   if (t1->tag != t2->tag) {
     return false;