Introduce Search namespace

Move global search-related variables under "Search" namespace.

As a side effect we can move uci_async_command() and
wait_for_stop_or_ponderhit() away from search.cpp

No functional change.

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

src/main.cpp

@@ -53,7 +53,7 @@ int main(int argc, char* argv[]) {
   init_bitboards();
   Position::init();
   kpk_bitbase_init();
-  init_search();
+  Search::init();
   Threads.init();
 
 #ifdef USE_CALLGRIND