Dynamically-bound validation context.

Register a validation function to run against the linked node.

Record a validation result. The function accepts a multihash node id, a keyword validation type, a result key (:pass, :warn, :fail, or :error) and a message string.

Validate a full graph structure by walking the nodes and links with validation functions. Returns a map from node ids to information maps, which contain a :paths set and a :results vector with assertion result maps.