Rename Bitboards print to pretty

To align to same named Position function and
avoid using std::cout directly.

Also remove some stale <iostream> include while
there.

No functional change.

src/book.cpp

@@ -25,7 +25,6 @@
 
 #include <algorithm>
 #include <cassert>
-#include <iostream>
 
 #include "book.h"
 #include "misc.h"