digest: cleanup digester and verifier creation

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

registry/storage/blobwriter.go

@@ -236,7 +236,7 @@ func (bw *blobWriter) validateBlob(ctx context.Context, desc distribution.Descri
 		if !verified {
 			digester := digest.Canonical.New()
 
-			digestVerifier, err := digest.NewDigestVerifier(desc.Digest)
+			digestVerifier, err := desc.Digest.Verifier()
 			if err != nil {
 				return distribution.Descriptor{}, err
 			}

registry/storage/filereader_test.go

@@ -41,7 +41,7 @@ func TestSimpleRead(t *testing.T) {
 		t.Fatalf("error allocating file reader: %v", err)
 	}
 
-	verifier, err := digest.NewDigestVerifier(dgst)
+	verifier, err := dgst.Verifier()
 	if err != nil {
 		t.Fatalf("error getting digest verifier: %s", err)
 	}