Add payload and signatures method to SignedManifest

To provide easier access to digestible content, the paylaod has been made
accessible on the signed manifest type. This hides the specifics of the
interaction with libtrust with the caveat that signatures may be parsed twice.

We'll have to have a future look at the interface for manifest as we may be
making problematic architectural decisions. We'll visit this after the initial
release.

Signed-off-by: Stephen J Day <stephen.day@docker.com>

storage/revisionstore.go

@@ -79,13 +79,8 @@ func (rs *revisionStore) get(revision digest.Digest) (*manifest.SignedManifest,
 // put stores the manifest in the repository, if not already present. Any
 // updated signatures will be stored, as well.
 func (rs *revisionStore) put(sm *manifest.SignedManifest) (digest.Digest, error) {
-	jsig, err := libtrust.ParsePrettySignature(sm.Raw, "signatures")
-	if err != nil {
-		return "", err
-	}
-
 	// Resolve the payload in the manifest.
-	payload, err := jsig.Payload()
+	payload, err := sm.Payload()
 	if err != nil {
 		return "", err
 	}
@@ -103,7 +98,7 @@ func (rs *revisionStore) put(sm *manifest.SignedManifest) (digest.Digest, error)
 	}
 
 	// Grab each json signature and store them.
-	signatures, err := jsig.Signatures()
+	signatures, err := sm.Signatures()
 	if err != nil {
 		return "", err
 	}