WIP: EC + ECDSA backend

This PR will need splitting up a lot. Mostly here for CI :)

TODO

- [ ] More edge cases tests
- [x] Make digest truncation work for 0.9.8?
- [x] Make sect163r1 work
- [ ] Update docs
- [x] Figure out how to name the named curve classes
- [ ] Should probably raise AlreadyFinalized
- [x] Figure our the backend.ec_supported interface properly.