Figure out coherent strategy around RSA keys

Per @davidben's comment [here](https://fuchsia-review.googlesource.com/c/mundane/+/486717/2/src/public/rsa/mod.rs#69):

> 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.

Start by reading the entire linked comment thread and the RSA implementation around src/public/rsa/mod.rs#69. Define which RSA private-key forms are supported, whether they use one type or separate types, and document the decisions in the RSA code; reorganize the implementation as appropriate.

Written by the indexing model from the issue text.