ad Ed25519 signatures

[warner]

At the 2nd Tahoe Summit, we hashed out a [plan ​https://tahoe-lafs.org/trac/tahoe-lafs/wiki/Summit2Day1#signatureconsensus] for Accounting that starts with adding Ed25519 signatures to pycryptopp. (Crypto++ doesn't yet include Ed25519, so the plan involves putting a separate copy of the Ed25519 C code into pycryptopp, adding Python bindings to it, and then replacing that C code with calls into Crypto++'s version once it shows up).

I've got a patch here against 0.5.29 that embeds a copy of my ​python-ed25519 library (version 0.7) into pycryptopp, including unit tests, power-on self-tests, and a subset of the Known Answer Tests that complete in about 1.0s .

[warner]

add cryptopp.publickey.ed25519