python/crypto/ed25519_tests: compute a point on Curve25519 from x
python/crypto/ed25519_tests: compute a point on Curve25519 from x

While at it, refactor the code in order to ease the access to Curve25519
primitives, eg. for X25519 Diffie-Hellman exchange.

fishilico/shared,fishilico/shared,fishilico/shared,fishilico/shared,fishilico/shared,fishilico/shared,fishilico/shared