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/12_pattern_matching.ex

@@ -1,4 +1,5 @@
 defmodule PatternMatching do
+  @moduledoc false
   use Koans
 
   @intro "PatternMatching"
@@ -94,6 +95,7 @@ defmodule PatternMatching do
   end
 
   defmodule Animal do
+    @moduledoc false
     defstruct [:kind, :name]
   end
 
@@ -103,6 +105,7 @@ defmodule PatternMatching do
   end
 
   defmodule Plane do
+    @moduledoc false
     defstruct passengers: 0, maker: :boeing
   end