added memory freeing functions

exercises/03/vslc/src/vslc.c

@@ -23,7 +23,8 @@ main ( int argc, char **argv )
     options ( argc, argv );
 
     yyparse();  // Generated from grammar/bison, constructs syntax tree
-
+    yylex_destroy(); // Free heap used by flex
+    
     if ( print_full_tree )
         print_syntax_tree ();
     simplify_syntax_tree ();    // In tree.c