add an ADL interface type

As a method superset of Node, with the Substrate method added on top.
This way, we can generally say that any ADL implementation should
include this method to allow for easy encoding over the wire.

Reify, the opposite of Substrate, is notably missing here. This is
because it's generally a top-level function, not a method. We might want
to rethink this disparity in the future.

node.go

@@ -162,6 +162,19 @@ type Node interface {
 	Prototype() NodePrototype
 }
 
+// ADL represents an Advanced Data Layout, a special kind of Node which
+// implements custom logic while still behaving like an IPLD node.
+//
+// For more details, see the docs at
+// https://github.com/ipld/specs/blob/master/schemas/authoring-guide.md.
+type ADL interface {
+	Node
+
+	// Substrate returns the underlying Data Model node, which can be used
+	// to encode an ADL's raw layout.
+	Substrate() Node
+}
+
 // NodePrototype describes a node implementation (all Node have a NodePrototype),
 // and a NodePrototype can always be used to get a NodeBuilder.
 //