[enh] SignigKey: add read and write methods for private key:

- cleaner separation of implementations between the software and the lib:
  - the client does not need to import libnacl
  - save() method already exists:
    - The new method allows its presence in the documentation
    - it could be find reading the example
    - as a beginner it’s not obvious to find it from the code
    as you have to understand super() and the inheritance schema in libnactl code.

- update save_and_load_private_key_file example