add: interfaces assertion and composition (wip)

interfaces/03-interfaces/main.go

@@ -22,10 +22,6 @@ func main() {
 	// printer(tetris).discount(.8)
 	store.print()
 
-	// type printer interface {
-	// 	print()
-	// }
-
 	// p can store a value of any type that has a `print()` method
 	var p printer
 	p = puzzle{title: "sidewinder", price: 10}