ENHANCED: Use crrl for Ed25519 signing and signature verification.

ENHANCED: Use crrl for Ed25519 signing and signature verification.

The main motivation for this change is the introduction of the newly
available predicate ed25519_seed_keypair/2, allowing to generate a key
pair from a given seed. In this way, a key pair can be dynamically
generated from (for example) a password, using crypto_password_hash/3
in combination with crypto_data_hkdf/4 to generate the seed. The
advantage of this method is that the private key need not be stored at
all anywhere.

It is not possible to add a corresponding feature to ring, since it is
closed as "not planned": https://github.com/briansmith/ring/issues/1003

I also used this opportunity to move more of the logic to Prolog. We
now have total control of the key pair representation, and I also
changed the representation to conform to the PKCS#8 v2 standard,
something that only later ring versions do, while still being
backwards compatible with tools that produce a wrong representation
including earlier ring versions.

Another great advantage we get from this change is that the Ed25519
predicates now also run on the 32-bit and WASM versions of Scryer.