Updates to the latest upstream changes in `curve25519-dalek`, including
using the new `EdwardsPoint::mul_base` API.
To keep the build deterministic, this also checks in Cargo.lock, which
pins `curve25519-dalek` to a particular git commit SHA which can be
updated using `cargo update -p curve25519-dalek`.
We can potentially remove `Cargo.lock` again after a crate release.
