Use curve25519-dalek from git; check in Cargo.lock (dalek-cryptogra…

…phy#260)

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.

.gitignore

@@ -1,5 +1,4 @@
 target
-Cargo.lock
 
 .cargo