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/board.h

@@ -20,6 +20,13 @@
 #include "piece.h"
 #include "bitboard.h"
 
+/* from human to machine */
+#define C2FILE(c)          (tolower(c) - 'a')
+#define C2RANK(c)          (tolower(c) - '1')
+/* from machine to human */
+#define FILE2C(f)          ((f) + 'a')
+#define RANK2C(r)          ((r) + '1')
+
 /* a square is defined as
  * rrrfff
  */