Digest::Adler32 and Digest::CRC32 do not inherit from ::Digest

[postmodern]

The ::Digest abstract class represents an API for calculating/updating/finalizing digests (originally based off of Ruby's Digest::Base class). However, Digest::Adler32 and Digest::CRC32 do not inherit from ::Digest. However, OpenSSL::Digest, Crystal::Digest::MD5, Crystal::Digest::SHA1 do inherit from ::Digest.

I feel like every type of digest algorithm under Digest:: should inherit from ::Digest and implement it's API.

[bcardiff]

FTR This was discussed briefly in #8881

[straight-shoota]

Changing the type from module to class is technically a breaking change. It would only break code where someone re-opened the stdlib type. I think that's acceptable.

I'm wondering if we should also deprecate the legacy API methods. They provide a different interface based on UInt32 (instead of generic Bytes as in the Digest API). Not sure if that's that much useful, but it probably doesn't hurt to keep it around.