x/crypto/ssh: add support for ed25519 keys

Added support for parsing the "new" openssh private key format.
(ed25519 keys only in this format for now)

Signing and verifying functions now work with ed25519 keys.

ed25519 can now be accepted by the server to authenticate a client.

ed25519 can now be accepted by a client as a server host key.

Related documentation used:
https://www.ietf.org/archive/id/draft-bjh21-ssh-ed25519-02.txt

Change-Id: I84385f24d666fea08de21f980f78623f7bff8007
Reviewed-on: https://go-review.googlesource.com/22512
Reviewed-by: Han-Wen Nienhuys <hanwen@google.com>
Run-TryBot: Han-Wen Nienhuys <hanwen@google.com>
9 files changed