Re-implement next_k_array; add k_array_rank

[oyamad]

1. Re-implement next_k_array which is now based on "Algotirhm T" in Generating All Combinations by Knuth.

2. Add k_array_rank and k_array_rank_jit:

   • k_array_rank works with pure Python's arbitrary precision integer.
   • k_array_rank_jit is Numba jit compiled and limited by the range of np.intp.

3. _next_k_array in support_enumeration is replaced by the new next_k_array:

   • The previous _next_k_array is implemented by "Gosper's hack" (see game_theory: Add support_enumeration #263 (comment)) which has a limit in the size of the input array by the number of bits of np.int.
   • The new next_k_array has no such limitation.
   • The performance does not change almost at all.

[coveralls]

Coverage increased (+0.02%) to 94.878% when pulling a215f1f on k_array into 475650b on master.

[mmcky]

thanks @oyamad I like the approach to using util/combinatorics. I will merge later this afternoon.