Reposition MapSets koans. Reposition some of the koans to make it clear why MapSets are interesting.

lib/koans/10_structs.ex

@@ -1,48 +0,0 @@
-defmodule Structs do
-  use Koans
-
-  @intro "Structs"
-
-  defmodule Person do
-    defstruct [:name, :age]
-  end
-
-  koan "Structs are defined and named after a module" do
-    person = %Person{}
-    assert person == ___
-  end
-
-  koan "Unless previously defined, fields begin as nil" do
-    nobody = %Person{}
-    assert nobody.age == ___
-  end
-
-  koan "You can pass initial values to structs" do
-    joe = %Person{name: "Joe", age: 23}
-    assert joe.name == ___
-  end
-
-  koan "Update fields with the cons '|' operator" do
-    joe = %Person{name: "Joe", age: 23}
-    older = %{joe | age: joe.age + 10}
-    assert older.age == ___
-  end
-
-  defmodule Plane do
-    defstruct passengers: 0, maker: :boeing
-  end
-
-  def plane?(%Plane{}), do: true
-  def plane?(_), do: false
-
-  koan "Or onto the type of the struct itself" do
-    assert plane?(%Plane{passengers: 417, maker: :boeing}) == ___
-    assert plane?(%Person{}) == ___
-  end
-
-  koan "Struct can be treated like maps" do
-    silvia = %Person{age: 22, name: "Silvia"}
-
-    assert Map.fetch(silvia, :age) == ___
-  end
-end