Move zobrist keys out of Position

Are used by Position but do not belong to that class,
there is only one instance of them (that's why were
defined as static), so move to a proper namespace instead.

No functional change.

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

src/main.cpp

@@ -34,7 +34,7 @@ int main(int argc, char* argv[]) {
 
   UCI::init(Options);
   Bitboards::init();
-  Position::init();
+  Zobrist::init();
   Bitbases::init_kpk();
   Search::init();
   Threads.init();