Make X25519_public_from_private() internally reachable.

Rework ED25519 API.

BoringSSL implemented a compound private key, which includes a copy of the
public key as a performance optimisation for signing. However, this does
not readily match with how EVP

Rework ED25519 API.

BoringSSL implemented a compound private key, which includes a copy of the
public key as a performance optimisation for signing. However, this does
not readily match with how EVP works, makes the ED25519 API inconsistent
with the X25519 API, diverges from th RFC and does not align with the
OpenSSL API. Instead, the caller can readily compute the public key and
pass this in to the signing process.

ok tb@

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Refactor/split ED25519_keypair.

This brings in ED25519_keypair_from_seed() from BoringSSL commit
c034e2d3ce16, which ED25519_keypair then wraps. This reduces differences
between us and BoringSSL.

$OpenBSD$

Use __{BEGIN,END}_HIDDEN_DECLS to avoid exporting the internal symbols

ok jsing@

Add support for X25519.

This brings in code from BoringSSL, which is mostly taken from SUPERCOP.

ok beck@ bcook@