Minimize struct copying

by using pointer func receivers and slices of struct pointers where possible.

command_test.go

@@ -42,13 +42,13 @@ func TestCommandFlagParsing(t *testing.T) {
 		err := command.Run(context)
 
 		expect(t, err, c.expectedErr)
-		expect(t, []string(context.Args()), c.testArgs)
+		expect(t, context.Args().Slice(), c.testArgs)
 	}
 }
 
 func TestCommand_Run_DoesNotOverwriteErrorFromBefore(t *testing.T) {
 	app := NewApp()
-	app.Commands = []Command{
+	app.Commands = []*Command{
 		{
 			Name: "bar",
 			Before: func(c *Context) error {
@@ -75,11 +75,11 @@ func TestCommand_Run_DoesNotOverwriteErrorFromBefore(t *testing.T) {
 
 func TestCommand_OnUsageError_WithWrongFlagValue(t *testing.T) {
 	app := NewApp()
-	app.Commands = []Command{
+	app.Commands = []*Command{
 		{
 			Name: "bar",
 			Flags: []Flag{
-				IntFlag{Name: "flag"},
+				&IntFlag{Name: "flag"},
 			},
 			OnUsageError: func(c *Context, err error, _ bool) error {
 				if !strings.HasPrefix(err.Error(), "invalid value \"wrong\"") {