Elligator script: use x25519_pk test vectors

Elligator script: use x25519_pk test vectors

We're now reading the `x25519_pk.all.vec` generated by Libsodium in
`x25519.c`, to make sure scalarmult is correctly implemented in the
Python script.

While we're at it, we also use them to generate Elligator 2 vectors.
Any addition to the X25519 public key generation will automatically
benefit Elligator 2 as well

TODO: update the makefile to make sure the vectors are generated before
we run `elligator.py`