Add credo to the project and:

- Run mix credo --all to identify possible code optimizations
 - Resolve most of the errors generated by credo such as:
   - Numbers larger than 9999 should be written with underscores: 58_127
   - Modules should have a @moduledoc tag
   - Comparison will always return true

lib/koans/10_structs.ex

@@ -1,9 +1,11 @@
 defmodule Structs do
+  @moduledoc false
   use Koans
 
   @intro "Structs"
 
   defmodule Person do
+    @moduledoc false
     defstruct [:name, :age]
   end
 
@@ -35,10 +37,12 @@ defmodule Structs do
   end
 
   defmodule Plane do
+    @moduledoc false
     defstruct passengers: 0, maker: :boeing
   end
 
   defmodule Airline do
+    @moduledoc false
     defstruct plane: %Plane{}, name: "Southwest"
   end