Introduce StateInfo instead of UndoInfo

We don't backup anymore but use the renamed StateInfo
argument passed in do_move() to store the new position
state when doing a move.

Backup is now just revert to previous StateInfo that we know
because we store a pointer to it.
Note that now backing store is up to the caller, Position is
stateless in that regard, state is accessed through a pointer.

This patch will let us remove all the backup/restore copying,
just a pointer switch is now necessary.

Note that do_null_move() still uses StateInfo as backup.

No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>

src/uci.cpp

@@ -219,12 +219,12 @@ namespace {
         if (token == "moves")
         {
             Move move;
-            UndoInfo u;
+            StateInfo st;
             while (!uip.eof())
             {
                 uip >> token;
                 move = move_from_string(RootPosition, token);
-                RootPosition.do_move(move, u);
+                RootPosition.do_move(move, st);
                 if (RootPosition.rule_50_counter() == 0)
                     RootPosition.reset_game_ply();
             }