Third-Party Cryptographic Implementations

[Joannis]

For implementing RSA support, which will not be landing in the main library as discussed, we'll need to support external implementations for at least public-private keys. I'd also recommend looking into the same for the symmetric encryption as seen in #48

[Lukasa]

I think a good idea here is to try to throw together a few implementations and see what the common API surface needs to be, and then to offer that as an extension point. We have some time to workshop APIs here.

[Lukasa]

@gwynne this seems like something you'd find fun!