Add digest trait implementation

[kmod-midori]

This allows primitives in this crate to be used with things like Hmac (I tried to do HMAC-MD5 directly within CNG, but has no success as for now, so opted for this).

[emgre]

A few comments that I will address shortly:

• Migrate to digest 0.10.4
  • A quick glance suggests that it's not that big of a deal to update this PR to 0.10
• Ideally, I would like the digest_trait module to be private, similarly to how block_cipher_trait was done.
  • For a first pass, we could skip the Fixed[...] traits and only implement the basic digest traits directly on Hash
  • On a second pass, we could add hard types for each hashing algorithm and implement the Fixed[...] traits
• We should also provide the MAC algorithms through the digest library.

[emgre]

This will be done after the 0.5 release.