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/bitboard.h

@@ -26,7 +26,7 @@
 namespace Bitboards {
 
 void init();
-void print(Bitboard b);
+const std::string pretty(Bitboard b);
 
 }