Improvements to the `ipns` package

This is a tracking issue to improve the `ipns` package, such as cleaning the verification process, types, etc. This can be done at the same time of https://github.com/ipfs/specs/issues/376 for example, or not.

## Things to take into account

- https://github.com/ipfs/specs/issues/376
- https://github.com/ipfs/go-ipns/issues/13 (link specs)