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>