Make use of the Digest framework of Ruby.

Ruby has an API abstraction layer for the Digest library family, to make it easy to implement a new hashing library that automatically share implementation of common features with existing ones.  The first commit makes digest-sha3-ruby officially part of the Digest family by making use of the layer, retiring `digest/helper`.

The second commit is for portability and the third is to fix the error class for argument errors.

All the existing tests should pass.  Thanks for your consideration.