Port implementation of ed25519

[pipermerriam]

Note: This work probably should not be done until we're sure ed25519 is going to be implemented.

What is wrong?

EIP665, if adopted will add a new pre-compile for ed25519

How can it be fixed

We can lift the implementation from this PR into pyethereum.

• [WIP] add ed25519 precompile (EIP665) pyethereum#862

We should probably look into supporting a pure-python implementation as well as using a fast implementation if available similar to how we handle secp256k1 signatures.