Improve type specifications and documentation

- Added missing type specifications for Hello function and rules - Updated documentation for the Deserialize protocol -
Cleaned up IBAN validation function documentation - Enhanced test fixture generation with clearer parsing and error
messages

lib/iban_ex.ex

@@ -12,6 +12,7 @@ defmodule IbanEx do
       :world
 
   """
+  @spec hello() :: :world
   def hello do
     :world
   end