ed25519ll is a low-level wrapper for the Ed25519 public key signature
system. It uses Extension() to compile a shared library that is not a
Python extension module, and then uses ctypes to talk to the library. With
luck it will only be necessary to compile ed25519ll once for each
platform, reusing its shared library across Python versions.

This wrapper also contains a reasonably performat pure-Python
fallback. Unlike the reference implementation, the Python implementation
does not contain protection against timing attacks.