Use curve25519-dalek-ng as dependency instead

[jsdanielh]

The former curve25519-dalek repository is inactive/not longer
maintained and the repository was forked and is maintained by the
zcrypto organization:
https://github.com/zkcrypto/curve25519-dalek-ng.
Now the crate is called curve25519-dalek-ng and its latest version
is 4.

[dconnolly]

We decided not to switch to curve25519-dalek-ng in January, and it looks like there haven't been any updates to it since then, so we'll stick with curve25519-dalek for the time being.

[dconnolly]

The former curve25519-dalek repository is inactive/not longer
maintained and the repository was forked and is maintained by the
zcrypto organization:
https://github.com/zkcrypto/curve25519-dalek-ng.
Now the crate is called curve25519-dalek-ng and its latest version
is 4.

Also curve25519-dalek is not inactive/unmaintained, the latest changes and major version alpha release were 22 days ago: dalek-cryptography/curve25519-dalek@076cf34

[jsdanielh]

We decided not to switch to curve25519-dalek-ng in January, and it looks like there haven't been any updates to it since then, so we'll stick with curve25519-dalek for the time being.

Oh Ok. Sorry, I didn't check closed PRs and wasn't aware of the decision to stick with curve25519-dalek.