Move stop signal to Threads

Instead of having Signals in the search namespace,
make the stop variables part of the Threads structure.
This moves more of the shared (atomic) variables towards
the thread-related structures, making their role more clear.

No functional change

Closes #1149

src/search.h

@@ -21,7 +21,6 @@
 #ifndef SEARCH_H_INCLUDED
 #define SEARCH_H_INCLUDED
 
-#include <atomic>
 #include <vector>
 
 #include "misc.h"
@@ -92,15 +91,6 @@ struct LimitsType {
   TimePoint startTime;
 };
 
-
-/// SignalsType struct stores atomic flags updated during the search, typically
-/// in an async fashion e.g. to stop the search by the GUI.
-
-struct SignalsType {
-  std::atomic_bool stop, stopOnPonderhit;
-};
-
-extern SignalsType Signals;
 extern LimitsType Limits;
 
 void init();