Decoupled crypto backends (the rest)

[sulami]

This is following up on #410 with the rest of the decoupled implementations, and addressing most of the review comments (I think?).

There are still some todos left for documentation, which I haven't filled in yet.

I've verified that tests & clippy pass with all combinations of features, but a lot of this I only know enough about to be dangerous, so I'd appreciate close looks at the actual implementations to make sure e.g. I've picked the correct library functions in each case.