Better browser builds and ES6 module support

Ideally, we'd take a page from the https://github.com/multiformats/js-multiformats book and support cjs, esm, etc builds via rollup and friends to support multiple platforms/use-cases. Since this is designed to be a light-weight alternative to other ipds-based libraries, it should have a really _low_ barrier to usage out of the box. In fact, moving forward, this should be a pretty critical requirement. Further (planned) dependency reduction will help here significantly, but we need a sustainable path to ESM builds that makes it really easy to consume this lib.

See also #73 and #74