Move Adler32 and CRC32 to Digest

[bcardiff]

Adler32 and CRC32 interface are kept unmodified and hence they don't implement Digest::Base, but their results are 32-bit fixes intead of a slice/string; plus they support the combine alternative. I think the current API while simple is enough.

The top-level versions are deprecated.

[asterite]

docs_main.cr should be updated as well

[bcardiff]

The files are still there. And digest is included with require "./digest/**".

When the deprecated are effectively dropped, yes, the docs_main.cr should be updated.

[straight-shoota]

I agree on moving Adler32 and CRC32 under a combined namespace. But I'm not sure they fit under the Digest namespace. As far as I am familiar with the matter, a (message) digest is usually associated with a cryptographic hash function (such as MD5 or SHA-1 which are currently available in Digest namespace; OpenSSL::Digest has even more).
Adler32 and CRC32 are not cryptographic hash functions but much weaker checksum/CRC.

We could still continue with this and accept there's some mismatch. Digest is not very specific and already established namespace in Crystal's stdlib.
Alternatively, we could consider a more broader term as top level namespace for hash functions. Hash is already taken, though ;)

[bcardiff]

I've found that many std-libs put CRC32 and Adler32 under Digest and none that uses another namespace ...

[asterite]

golang uses the hash namespace:

• https://golang.org/pkg/hash/adler32/
• https://golang.org/pkg/hash/crc32/

That said, I'm fine with digest

[ysbaddaden]

Some digest algorithms are cryptographically strong but that's hardly a requirement, otherwise both MD5 (broken) and SHA-1 (weak) wouldn't be digest algorithms anymore, which would be nonsense.

Don't use CRC32, Adler32, MD5 or SHA-1 for secure usages like verifying that a file wasn't tampered with. Use SHA-2, SHA-3 or BLAKE. But not all usages need to be secure (e.g. gzip, deflate).

This is similar to random number generators: some are them are cryptographically strong.

[ysbaddaden]

Sure we can say hash instead of digest, but the term would be confusing in Crystal (Hash).

[bcardiff]

I forgot to change the usages of them in the std-lib.
Rebased on master.

[whidbey]

i dont notice this change.but i dont' think is good idea to move A to B and move C to D 's breaking change like this.
thoght I admire your commits..but It's a wasting time i think... what I want is better "C" ..