Move to_char() and to_string() to notation

Where they better belong.

Also, this removes '#include <string>' from types.h, which reduces the amount of code to compile (every
translation unit includes types.h).

No functional change.

src/bitboard.h

@@ -21,6 +21,7 @@
 #ifndef BITBOARD_H_INCLUDED
 #define BITBOARD_H_INCLUDED
 
+#include <string>
 #include "types.h"
 
 namespace Bitboards {