ecc: Change keygrip computation for Ed25519+EdDSA.

* cipher/ecc.c (compute_keygrip): Rework.
* cipher/ecc-eddsa.c (_gcry_ecc_eddsa_ensure_compact): New.
* cipher/ecc-curves.c (_gcry_ecc_update_curve_param): New.
* tests/keygrip.c (key_grips): Add flag param and test cases for
Ed25519.

The keygrip for Ed25519+EdDSA has not yet been used - thus it is
possible to change it. Using the compact representation saves us the
recovering of x from the standard representation. Compacting is
basically free.