Add a utility function for converting path.Trace to a list

There are cases in which users of path.Trace want to do forward
iteration of the path. path.Trace does not permit it, because it's a
backward pointing linked list.

Instead of adding a ToList() function, we could have extended Trace to
implement Parser. But this is needlessly complex, because traces are
guaranteed to be well formed, and unlike Builder don't contain any ".."
components.

pkg/filesystem/path/trace.go

@@ -67,3 +67,19 @@ func (t *Trace) GetWindowsString() (string, error) {
 	t.writeToStringBuilder('\\', &sb)
 	return sb.String(), nil
 }
+
+// ToList returns the pathname components contained in the trace to a list.
+func (t *Trace) ToList() []Component {
+	count := 0
+	for tCount := t; tCount != nil; tCount = tCount.parent {
+		count++
+	}
+
+	components := make([]Component, count)
+	for count > 0 {
+		count--
+		components[count] = t.component
+		t = t.parent
+	}
+	return components
+}

pkg/filesystem/path/trace_test.go

@@ -59,4 +59,27 @@ func TestTrace(t *testing.T) {
 			testutil.RequireEqualStatus(t, status.Error(codes.InvalidArgument, "Invalid pathname component \"b?\": Pathname component contains reserved characters"), err)
 		}
 	})
+
+	t.Run("ToList", func(t *testing.T) {
+		var p1 *path.Trace
+		require.Empty(t, p1.ToList())
+
+		p2 := p1.Append(path.MustNewComponent("a"))
+		require.Equal(t, []path.Component{
+			path.MustNewComponent("a"),
+		}, p2.ToList())
+
+		p3 := p2.Append(path.MustNewComponent("b"))
+		require.Equal(t, []path.Component{
+			path.MustNewComponent("a"),
+			path.MustNewComponent("b"),
+		}, p3.ToList())
+
+		p4 := p3.Append(path.MustNewComponent("c"))
+		require.Equal(t, []path.Component{
+			path.MustNewComponent("a"),
+			path.MustNewComponent("b"),
+			path.MustNewComponent("c"),
+		}, p4.ToList())
+	})
 }