spki: adds a der::Writer adapter for Digest

[baloo]

This is used to hash der serialization of certificates.

This is stacked on #1689

[tarcieri]

Hmm, I get the utility (ssh-encoding does something similar), but I'm not super wild about der depending on digest.

Will have to think about it.

[baloo]

Yeah neither direction (digest depending on der, or der on digest) work great imho.
I guess this could be placed in a 3rd party crate but I don’t have a good idea on which one.

[tarcieri]

It's already in spki, right? We could potentially expose it from there (albeit a bit odd)

[baloo]

There is a more specialized version of it there. But yeah, this is a better option