Refactor
Refactor specs of HexadecimalString and Set01::Challenge01 for clarity
Configure Guard to run all specs at first
Configure Guard to focus on failing specs until they pass
Solve Set 1, Challenge 3

Implement #length, #to_bytes, and #to_s on Base64String and
HexadecimalString.
Tune up descriptions within specs of Base64String and HexadecimalString