Figure out coherent strategy around RSA keys

[joshlf]

Per @davidben's comment here:

I think you should probably think about what things you want to support and build an API around that.

• What flavors of RSA private keys do you believe in? Just CRT? n/e/d as well?
• If you support multiple, do you want to jam them into one type or separate types? (Separate might make more sense given only CRT is serializable.)

Read the entire comment thread for more context.

We should answer these questions, document them clearly in our RSA code, and then update the organization of our RSA implementation as appropriate.