Revise Octavo `digest` structure about octavo HOT 6 CLOSED

My personal preference would be as follows:

use octavo::digest::Sha1;
use octavo::digest::sha2::Sha512;
use octavo::digest::sha3::Sha256;

This fits my expectations as a user: I've never heard of the the SHA3 versions referred to as - e.g. sha3512. SHA1 is its own algorithm, whereas SHA2/3 are families of algorithms. And it's still easy to use both in the same program:

use octavo::digest::{sha2, sha3};
let digest1 = sha2::Sha512::default();
let digest2 = sha3::Sha512::default();

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Me too, but as SHA-3 is fairly new I do not hear much about it. SHA-2 functions are commonly refereed as SHA-XXX not SHA2-XXX and only tool that support SHA-3 under that name (that I know) is rhash which refers to it as SHA3-XXX.

To make it more weird and complicated SHA-1 and SHA-2 was described in the same document FIPS 180-4 and SHA-2 hash family was refereed as SHA-XXX there, this make impression that SHA-1 and SHA-2 (namely SHA-XXX) are related and SHA-3 is whole new hash function family.

EDIT:

Also NIST on their page refers to SHA-2 family as SHA-XXX and to SHA-3 as SHA3-XXX

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@Dr-Emann suggestion looks good to me, when in doubt naming should be obvious even if we need one more level for the namespace.

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@arthurprs it also seems right to me, but I don't know if Sha1 should be reexported. It could be both, but I am still not sure.

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This also leave us with whole MD family. It should be moved into one module named octavo::digest::md or should be left as is?

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FIxed by #53

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