Expose direct access API for Ed25519.

zap stray space (CRITICAL!)

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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Enable Ed25519 internal to libcrypto.

Based on a diff from tb@

$OpenBSD$

Define X25519_KEY_LENGTH and use it so we have fewer magic numbers.

ok beck@

Add support for X25519.

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

ok beck@ bcook@