sq_attackers() + others (see dedails). Ready for move do/undo ?

- add many "const" in func parameters
- attack.c: sq_attackers()
- move print_board_raw from position.c to to board.c
- move some fen_check() tests to pos_check()
- add REL_RANK() macro. TODO: add one more for bitboards
- fen.c: more tests for FEN validity
- position.c: add pos_checkers() and pos_check()
- tests: add common-test.h (for shared FEN positions access)

src/attack.h

@@ -17,6 +17,7 @@
 #include "chessdefs.h"
 #include "bitboard.h"
 
-extern bitboard_t sq_attackers(pos_t *pos, square_t sq, color_t c);
+extern bitboard_t sq_attackers(const pos_t *pos, const square_t sq, const color_t c);
+extern bitboard_t sq_attackers_all(const pos_t *pos, const square_t sq);
 
 #endif