Avoid mutation of ed25519 key passed into keyToDidDoc.

[msporny]

Added

• Add keypairs property that indexes verification methods.

Changed

• Avoid mutation of ed25519 key passed into keyToDidDoc.
• Set id field for keypairs.

This is being driven by the ezcap work, I'm trying to get to a nice pattern to create the ezcap client:

const baseUrl = 'https://zcap.example/';
const didDocument = await didKeyDriver.generate();
const delegationKeypair = didDocument.keypairs.capabilityDelegation[0];
const invocationKeypair = didDocument.keypairs.capabilityInvocation[0];
zcapClient = new zcapClient({baseUrl, httpsAgent, delegationKeypair, invocationKeypair});

[msporny]

Closing in favor of https://github.com/digitalbazaar/did-method-key-js/pull/34