Update `rsa` dependency (and use `x25519-dalek` prerelease)

• #448 (closed) and !607 (closed) 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.

closes #448 (closed)