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/blanks.ex

@@ -1,4 +1,5 @@
 defmodule Blanks do
+  @moduledoc false
   def replace(ast, replacements) do
     replacements = List.wrap(replacements)