Move sigils to later in the lessons

While doing this, I also discovered that
there is also a reference to lists in numbers, but
that might a bit easier for someone to grasp, given
the hint that the koan gives, and the output they
see when they run it.

lib/koans/10_structs.ex

@@ -0,0 +1,67 @@
+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
+
+  defmodule Airline do
+    defstruct plane: %Plane{}, name: "Southwest"
+  end
+
+  koan "Use the put_in macro to replace a nested value" do
+    airline = %Airline{plane: %Plane{maker: :boeing}}
+    assert put_in(airline.plane.maker, :airbus) == ___
+  end
+
+  koan "Use the update_in macro to modify a nested value" do
+    airline = %Airline{plane: %Plane{maker: :boeing, passengers: 200}}
+    assert update_in(airline.plane.passengers, &(&1 + 2)) == ___
+  end
+
+  koan "Use the put_in macro with atoms to replace a nested value in a non-struct" do
+    airline = %{plane: %{maker: :boeing}, name: "Southwest"}
+    assert put_in(airline[:plane][:maker], :cessna) == ___
+  end
+end