Error `Hypercore key should be 32 bytes` reduces the usefulness of custom `opts.crypto`

[Nuhvi]

Checking for a 32 bytes publicKey makes is it impossible to use secp256k1 compressed publicKey https://github.com/hypercore-protocol/hypercore-next/blob/master/index.js#L35

Possible solution:

    if (!opts.crypto && key && key.byteLength !== 32) {
      throw new Error('Hypercore key should be 32 bytes')
    }

[mafintosh]

Yea good solution, wanna PR?

[Nuhvi]

On it.