Move storage package to use Digest type

Mostly, we've made superficial changes to the storage package to start using
the Digest type. Many of the exported interface methods have been changed to
reflect this in addition to changes in the way layer uploads will be initiated.

Further work here is necessary but will come with a separate PR.

storage/paths_test.go

@@ -1,6 +1,10 @@
 package storage
 
-import "testing"
+import (
+	"testing"
+
+	"github.com/docker/docker-registry/digest"
+)
 
 func TestPathMapper(t *testing.T) {
 	pm := &pathMapper{
@@ -15,13 +19,13 @@ func TestPathMapper(t *testing.T) {
 		{
 			spec: layerLinkPathSpec{
 				name:   "foo/bar",
-				tarSum: "tarsum.v1+test:abcdef",
+				digest: digest.Digest("tarsum.v1+test:abcdef"),
 			},
 			expected: "/pathmapper-test/repositories/foo/bar/layers/tarsum/v1/test/abcdef",
 		},
 		{
 			spec: layerIndexLinkPathSpec{
-				tarSum: "tarsum.v1+test:abcdef",
+				digest: digest.Digest("tarsum.v1+test:abcdef"),
 			},
 			expected: "/pathmapper-test/layerindex/tarsum/v1/test/abcdef",
 		},