Introduce Eval namespace

Wrap evaluation related stuff and reshuffle
a bit the code.

No functional change.

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

src/uci.cpp

@@ -105,10 +105,7 @@ void uci_loop() {
           pos.flip_me();
 
       else if (token == "eval")
-      {
-          EvalRootColor = pos.side_to_move();
-          cout << trace_evaluate(pos) << endl;
-      }
+          cout << Eval::trace(pos) << endl;
 
       else if (token == "key")
           cout << "key: " << hex     << pos.key()