Add koans for floats, rename to numbers koans, reposition on the list

lib/koans/10_structs.ex

@@ -0,0 +1,48 @@
+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