Arti no longer builds because of update to x25519-dalek prerelease

@juga reported that an application built on top of some of our crates started to not compile.

The root cause is as follows:

• Our dependency on x25519-dalek is specified as x25519-dalek = "2.0.0-pre.1". At the time, that was presumably the most recent version.
• x25519-dalek have published a 2.0.0-rc.2 which had a breaking change. https://github.com/dalek-cryptography/x25519-dalek/blob/main/CHANGELOG.md#2x-series
• cargo considers that 2.0.0-rc.2 satisfied 2.0.0-pre.1 even though the documentation says that crates are likely to have breaking changes. https://doc.rust-lang.org/cargo/reference/resolver.html#pre-releases. This is clearly a bug in cargo that will probably never be fixed.

Conclusion: when specifying a pre-release version, one should write something like x25519-dalek = "=2.0.0-pre.1".

I think we should do an emergency point release of tor-llcrypto to crates.io so that things still build for people. I will make a branch starting at arti-v1.1.3 for this purpose.

CC @nickm

error[E0432]: unresolved import `x25519_dalek::StaticSecret`
  --> /home/user/.cargo/registry/src/github.com-1ecc6299db9ec823/tor-llcrypto-0.4.3/src/pk.rs:16:70
   |
16 |     pub use x25519_dalek::{EphemeralSecret, PublicKey, SharedSecret, StaticSecret};
   |                                                                      ^^^^^^^^^^^^
   |                                                                      |
   |                                                                      no `StaticSecret` in the root
   |                                                                      help: a similar name exists in the module: `SharedSecret`