Import ed25519 from openssh-6.6p1

Minor modifications were done by me to remove the OpenSSH dependency /
make it freestanding.
ed25519: Use SHA-512 from OpenSSL

No idea why the OpenSSH guys didn't do that already and shipped it with
another copy of SHA-512.
Improve ed25519

 * Don't copy the message when signing
 * Don't copy the message / set it to zero when verifying
 * Improve API
 * General cleanup
More ed25519 improvements

* Get rid of crypto_uint32
* Get rid of crypto_api.h
* Integrate verify.c into ed25519.c
* Add ed25519.h header to use ed2559.c