In the Python bindings, move string serialization to Rust land

Since Python is not doing anything with the bytes it receives but encoding them to string, it's a good idea to move the string encoding to Rust layer for faster conversion and simpler usage from the Python client side perspective 

See example usage of this:
https://github.com/LDiazN/anoncred/blob/a6071ee5f79dc802445ab9e8f707924c6af4ea74/tests/test_creds.py#L11