Get rid of PyNaCl and use base58 module

Use proper libraries: hashlib.sha256 stdlib and base58

• Remove Base58 code, use base58 Python module
  • Add base58 as a direct dependency to Silkaj? It is already a dependency of DuniterPy. Silkaj will still use DuniterPy and DuniterPy will still use base58. Should allow to not have to remove base58 from Silkaj's pyproject.toml when base58 is not longer directly used.
• Remove direct PyNaCl dependency from Poetry and the README.md.
• Instead, use sha256 from hashlib stdlib