new pos/bb funcs, legal(), better castling gen, etc. [see commit details]

- new sq_pinners
- new pseudo_is_legal() (unfinished)
- improve castling pseudo move gen

- more position and lower level bitboard helper funcs:
  - pos_{_occ,between_occ,between_count,pinners}
  - bb_{rank,file,rel_rank,_sq_aligned,_sq_between)
- rename some bitboard globals
- replace bb ranks/files enums with defines (issue with clang)
  -> Need to find a way to use enum safely
- tests:
  - add common-test.h
  - new attack-test.c

src/attack.h

@@ -19,5 +19,5 @@
 
 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);
-
+extern bitboard_t sq_pinners(const pos_t *pos, const square_t sq, const color_t c);
 #endif