crypto/ed25519: promote from golang.org/x/crypto/ed25519

The crypto/tls and crypto/x509 APIs leak PublicKey and PrivateKey types,
so in order to add support for Ed25519 certificates we need the ed25519
package in the stdlib.

It's also a primitive that's reasonable to use directly in applications,
as it is a modern, safe and fast signing algorithm, for which there
aren't higher level APIs. (The nacl/sign API is limiting in that it
repeats the message.)

A few docs changes will come in a follow-up, and a CL will land on
golang.org/x/crypto/ed25519 to make it a type alias wrapper on Go 1.13+.

Updates #25355

Change-Id: I057f20cc7d1aca2b95c29ce73eb03c3b237e413f
Reviewed-on: https://go-review.googlesource.com/c/go/+/174945


Run-TryBot: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Adam Langley <agl@golang.org>