Add ED25519_keypair_from_seed.

This function allows callers to unpack an Ed25519 “seed” value, which is
a 32 byte value that contains sufficient information to build a public
and private key from.

Change-Id: Ie5d8212a73e5710306314b4f8a93b707665870fd
Reviewed-on: https://boringssl-review.googlesource.com/12040
Reviewed-by: David Benjamin <davidben@google.com>
Reviewed-by: Adam Langley <alangley@gmail.com>
Commit-Queue: Adam Langley <alangley@gmail.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>

include/openssl/curve25519.h

diff --git a/include/openssl/curve25519.h b/include/openssl/curve25519.h
index 8d73f33..19f9daa 100644
--- a/include/openssl/curve25519.h
+++ b/include/openssl/curve25519.h
@@ -85,6 +85,15 @@
                                   const uint8_t signature[64],
                                   const uint8_t public_key[32]);
 
+/* ED25519_keypair_from_seed calculates a public and private key from an
+ * Ed25519 “seed”. Seed values are not exposed by this API (although they
+ * happen to be the first 32 bytes of a private key) so this function is for
+ * interoperating with systems that may store just a seed instead of a full
+ * private key. */
+OPENSSL_EXPORT void ED25519_keypair_from_seed(uint8_t out_public_key[32],
+                                              uint8_t out_private_key[64],
+                                              const uint8_t seed[32]);
+
 
 /* SPAKE2.
  *