add memory pool for positions

src/eval.c

@@ -86,7 +86,8 @@ int main(int ac, char**av)
     debug_init(2);
     piece_pool_init();
     moves_pool_init();
-    pos = pos_create();
+    pos_pool_init();
+    pos = pos_get();
 
     if (ac == 1) {
         pos_startpos(pos);