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/move-gen.h

@@ -21,10 +21,7 @@
 #include "piece.h"
 #include "move.h"
 
-/**
- * gen_all_pseudomoves() - generate all pseudo moves
- *
- */
+bool pseudo_is_legal(pos_t *pos, move_t move);
 int gen_all_pseudomoves(pos_t *pos);
 
 #endif  /* MOVEGEN_H */