Refine Digest

[ksss]

• Should be return bot for methods of abstruct class. Use interface instead.
• The number of arguments of Digest::Class methods cannot be determined because they are delegated to the inherited initialize.
• Fix some types.

Good points

• Become a more accurate signatures.

Bad points

• Comments are effectively lost because annotations cannot be automatically written on the interface.

[soutaro]

Having -> bot types doesn't look a good idea, especially for Digest::Base. We may want to have Digest::Base type which might be one of the sub classes instances, and it all method calls will result in bot.

[ksss]

I understand. I would like to modify it with a different approach. I will close this PR.