Initial implementation of storage layer path mapper

We've added a path mapper to support simple mapping between path objects used
in the storage layer and the underlying file system. The target of this is to
ensure that paths are only calculated in a single place and their format is
separated from the data that makes up the path components.

This commit only includes spec implementation to support layer reads. Further
specs will come along with their implementations.

storage/paths_test.go

@@ -0,0 +1,45 @@
+package storage
+
+import "testing"
+
+func TestPathMapper(t *testing.T) {
+	pm := &pathMapper{
+		root: "/pathmapper-test",
+	}
+
+	for _, testcase := range []struct {
+		spec     pathSpec
+		expected string
+		err      error
+	}{
+		{
+			spec: layerLinkPathSpec{
+				name:   "foo/bar",
+				tarSum: "tarsum.v1+test:abcdef",
+			},
+			expected: "/pathmapper-test/repositories/foo/bar/layers/tarsum/v1/test/abcdef",
+		},
+		{
+			spec: layerIndexLinkPathSpec{
+				tarSum: "tarsum.v1+test:abcdef",
+			},
+			expected: "/pathmapper-test/layerindex/tarsum/v1/test/abcdef",
+		},
+		{
+			spec: blobPathSpec{
+				alg:    "sha512",
+				digest: "abcdefabcdefabcdef908909909",
+			},
+			expected: "/pathmapper-test/blob/sha512/ab/abcdefabcdefabcdef908909909",
+		},
+	} {
+		p, err := pm.path(testcase.spec)
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		if p != testcase.expected {
+			t.Fatalf("unexpected path generated: %q != %q", p, testcase.expected)
+		}
+	}
+}