- 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)
- bitboard.c: make attacks for knight/king
- square macros (BB, BBfile, BBrank) renamed sq_make, sq_file,
sq_rank, moved to board.h (and become temporarily inline funcs)
- different macros/defs moved to "correct place" (bitboard/board/piece):
board.[ch]: everything related to board/square
bitboard.[ch]: everything related to bitboards
piece.[ch]: everything related to pieces
