llcrypto: upgrade x25519-dalek.
This upgrades us to 2.0.0-rc.2, which is (according to crates.io) the latest in the not-quite-done-yet 2.0 series.
There is an argument to be made that we should switch to x25519-dalek-fiat
or x25519-dalek-ng
some other fork instead, but that is for another day. If we do, we might need to revert some of these API changes.
Related to #808 (closed), but that ticket mentions 2.0.0-rc.3, which crates.io hasn't heard of.