Added get_wordn, set_wordn functions; Merged lookup3.c; Exported BIT_MASK

Added:
  uint8_t  bit_array_get_wordn (const BIT_ARRAY* bitarr, bit_index_t start, int n)
  void bit_array_set_wordn (BIT_ARRAY* bitarr, bit_index_t start, uint64_t word, int n)

Merged lookup3.c into bit_array.c

Exported macros:
  word_t BIT_MASK(int nbits)
  word_t BIT_MASK_MERGE(word_t a, word_t b, int abits)