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/main.cpp

@@ -5,6 +5,7 @@
 #include <cstdio>
 #include <cstring>
 #include <iostream>
+#include <string>
 
 #include "executable_semantics/syntax_helpers.h"
 
@@ -23,5 +24,9 @@ int main(int argc, char* argv[]) {
       return 1;
     }
   }
+
+  if (argc > 2 && argv[2] == std::string("-trace")) {
+    Carbon::tracing_output = true;
+  }
   return yyparse();
 }