Right now this is just a wrapper over EVP_Digest and EVP_PKEY_sign. A
later change will introduce a sign_message hook to EVP_PKEY_METHOD which
Ed25519 and other single-shot-only algorithms can implement.
(EVP_PKEY_sign does not quite work for this purpose as all the other key
types believe EVP_PKEY_sign acts on a pre-hashed input.)

BUG=187

Change-Id: Ia4bbf61b25cc4a0d64bcb4364805fe9b5a6e829c
Reviewed-on: https://boringssl-review.googlesource.com/14447
Commit-Queue: David Benjamin <davidben@google.com>
Reviewed-by: Adam Langley <agl@google.com>