Reorganize eddsa, based on patch by Daiki Ueno.

* eddsa-internal.h (struct ecc_eddsa): New struct for eddsa
parameters.
* ed25519-sha512.c (_nettle_ed25519_sha512): New parameter struct.
* eddsa-expand.c (_eddsa_expand_key): Replace input
struct nettle_hash with struct ecc_eddsa, and generalize for
ed448. Update all callers.
* eddsa-sign.c (_eddsa_sign): Likewise.
* eddsa-verify.c (_eddsa_verify): Likewise.
* eddsa-compress.c (_eddsa_compress): Store sign bit in most
significant bit of last byte, as specified by RFC 8032.
* eddsa-decompress.c (_eddsa_decompress): Corresponding update.
Also generalize to support ed448, and make validity checks
stricter.
* testsuite/eddsa-sign-test.c (test_ed25519_sign): New function.
(test_main): Use it.
* testsuite/eddsa-verify-test.c (test_ed25519): New function.
(test_main): Use it.