Refactor ArrayTools to EnumerableTools.

Current spec status: 46 examples, 0 failures in 0.01913 seconds.
Implement IntegerTools#count_digits.

Current spec status: 58 examples, 0 failures in 0.02093 seconds.
Implement IntegerTools#digits.

Current spec status: 70 examples, 0 failures in 0.0216 seconds.