[curves] pure-python Ed25519 library for review

[D. J. Bernstein]

https://github.com/pyca/ed25519/blob/master/ed25519.py has a similar
goal. I haven't had a chance to look at any of the code.

Maybe the Python maintainers can be convinced to add a constant-time
string type to the language, with support for I/O and bignum arithmetic.
I'm saying "string type" rather than just "bignum type" because secret
keys (and plaintext messages!) need to be kept inside the firewall, not
exposed to hashing/deduplication/etc.

---Dan