Wrap it into npm library & add tests

[brusherru]

I've wrapped it into npm library (ready to publish and easy to use) and added tests.
Then it can be used like this:

import Ed25519 from '@spacemeshos/ed25519';

...
const keypair = await Ed25519.generateKeyPair(mySeed);
const message = Buffer.from('Hello World');
const signature = await Ed25519.sign(keypair.secretKey, message);
const isValid = await Ed25519.verify(keypair.publicKey, message, signature);

[fasmat]

LTGM, I'll merge it into the other branch and finish the change there 🙂