- Get rid of CryptoManager.
- Get rid of Crypto.
- Move all the Crypto instance methods to llarp::crypto functions.
(None of them needed to be methods at all, so this is simple).
- Move sodium/ntru initialization into static initialization.
- Add llarp::csrng, which is an available llarp::CSRNG instance which is
a bit easier than needing to construct a `CSRNG rng{};` in various
places.
- Various related small simplifications/cleanups.
