Added arena free macro for cleanup and clarity

tests/08_arena_exception.c

@@ -11,7 +11,7 @@ int main(void){
         a = ARENA(0,nbytes); /* try 1GB */
         if (!a.beg)
             RAISE(OOM);
-        free(a.beg);
+        ARENA_FREE(a);
     }
     EXCEPT(OOM) {
         /* handle memory failure gracefully */
@@ -20,7 +20,7 @@ int main(void){
     }
     FINALLY {
         /* cleanup if needed, runs always */
-        if (a.beg) free(a.beg);
+        if (a.beg) ARENA_FREE(a);
     }
     END_TRY;