WIP: open-pgp: Add a pure Rust backend
This is an exploration of what a pure Rust backend for Sequoia would look like. It is far from complete: most notably, support for the p256, p384, and p521 curves are missing. Likely to be more than a few bugs around too. Nonetheless, I thought I'd submit it in case someone wants to use this as a basis for #333 (closed).
I split the one piece that seemed like it was mergeable into its own PR, !942 (closed).