- arti#448 and arti!607 highlight an issue with upgrading `rsa`: namely,
the `x25519-dalek` version previously used has a hard dependency on
`zeroize` 1.3, which creates a dependency conflict.
- However, `x25519-dalek` version `2.0.0-pre.1` relaxes this dependency.
Reviewing the changelogs, it doesn't look like that version is
substantially different from the current one at all, so it should be
safe to use despite the "prerelease" tag.
- The new `x25519-dalek` version also bumps `rand_core`, which means we
don't have to use the RNG compat wrapper in `tor-llcrypto` as much.
closes arti#448
c2e2da5dc3