Cleanup includes

Reorder a few includes, include "position.h" where it was previously missing
and apply include-what-you-use suggestions. Also make the order of the includes
consistent, in the following way:

1. Related header (for .cpp files)
2. A blank line
3. C/C++ headers
4. A blank line
5. All other header files

closes https://github.com/official-stockfish/Stockfish/pull/4763
fixes https://github.com/official-stockfish/Stockfish/issues/4707

No functional change

src/movegen.cpp

@@ -16,9 +16,12 @@
   along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */
 
-#include <cassert>
-
 #include "movegen.h"
+
+#include <cassert>
+#include <initializer_list>
+
+#include "bitboard.h"
 #include "position.h"
 
 namespace Stockfish {