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Add a `Walker` structure with a simple interface to iterate and search inside DAGs, where a DAG need not be DAG of IPLD nodes but any graph of nodes meeting the `NavigableNode` interface.

Add a `NavigableIPLDNode` structure (encapsulating the IPLD `Node` interface) that implements the `NavigableNode` interface that allows to traverse a DAG of IPLD `Node`s with support for node promises.

License: MIT
Signed-off-by: Lucas Molas <schomatis@gmail.com>