Add exercise 2 from chapter 20

Author: Bernard Pietraga

chapter_20/macros_and_code_evalution_2.exs

@@ -0,0 +1,31 @@
+defmodule Times do
+  defmacro times_n(name_number) do
+    quote do:
+      def unquote(:"times_#{name_number}")(argument_integer), do:
+        unquote(name_number) * argument_integer
+  end
+end
+
+defmodule Test do
+  require Times
+
+  Times.times_n(3)
+  Times.times_n(4)
+end
+
+ExUnit.start
+
+defmodule TimesTest do
+  import Times
+  use ExUnit.Case
+
+  test "times_3 called with 4 returns 12" do
+    expected = Test.times_3(4)
+    assert expected == 12
+  end
+
+  test "times_4 called with 5 returns 20" do
+    expected = Test.times_4(5)
+    assert expected == 20
+  end
+end