Use a set to store SearchMoves

We just need to verify if a legal move is among the
SearchMoves, so we don't need a vector for this.

No functional change.

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

src/thread.cpp

@@ -431,7 +431,7 @@ void Thread::main_loop() {
 // the search to finish.
 
 void ThreadsManager::start_thinking(const Position& pos, const LimitsType& limits,
-                                    const std::vector<Move>& searchMoves, bool asyncMode) {
+                                    const std::set<Move>& searchMoves, bool asyncMode) {
   Thread& main = threads[0];
 
   lock_grab(&main.sleepLock);