Use bool(Bitboard b) instead of !!b (#1321)

The idiom !!b is confusing newcomers (e.g. Stefan needs explaining here https://groups.google.com/d/msg/fishcooking/vYqnsRI4brY/Gaf60QuACwAJ).

No functional change.

src/search.cpp

@@ -1115,7 +1115,7 @@ moves_loop: // When in check search starts from here
 
     const bool PvNode = NT == PV;
 
-    assert(InCheck == !!pos.checkers());
+    assert(InCheck == bool(pos.checkers()));
     assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
     assert(PvNode || (alpha == beta - 1));
     assert(depth <= DEPTH_ZERO);