go-ipld example

The new way of using IPLD with RPC to kubo was not clear to me. I didn’t know how I should manipulate ipld nodes, marshall/unmarshall them, or navigate through them. In this example, I do the following:

1. Use basicnode to unmarshall ipld data from codec to data structure.
2. Use a LinkSystem (wrapping rpc.Block() to make the following the links created by basicnode possible.
3. Use a selector to facilitate the traversal (without a bunch type casts) of a tree of multiple nodes.