New syntax for to-be-closed variables

The new syntax is <local *toclose x = f()>. The mark '*' allows other
attributes to be added later without the need of new keywords; it
also allows better error messages.  The API function was also renamed
('lua_tobeclosed' -> 'lua_toclose').

ltests.c

@@ -1550,8 +1550,8 @@ static struct X { int x; } x;
       int i = getindex;
       return lua_yieldk(L1, nres, i, Cfunck);
     }
-    else if EQ("tobeclosed") {
-      lua_tobeclosed(L);
+    else if EQ("toclose") {
+      lua_toclose(L);
     }
     else luaL_error(L, "unknown instruction %s", buff);
   }