Add NodeBuilder to Node interface.

(... offically.  Lots of docs have probably already been stating that
this is there.  Now it actually... is.)
Signed-off-by: Eric Myhre <hash@exultant.us>

impl/bind/boundNode.go

@@ -103,6 +103,10 @@ func (n Node) AsLink() (v ipld.Link, _ error) {
 	return
 }
 
+func (n Node) NodeBuilder() ipld.NodeBuilder {
+	panic("NYI")
+}
+
 func (n Node) Keys() ipld.KeyIterator {
 	panic("NYI")
 }

impl/free/freeNode.go

@@ -65,6 +65,10 @@ func (n *Node) AsLink() (v ipld.Link, _ error) {
 	return n._link, expectTyp(ipld.ReprKind_Link, n.kind)
 }
 
+func (n *Node) NodeBuilder() ipld.NodeBuilder {
+	return nodeBuilder{n}
+}
+
 func (n *Node) Keys() ipld.KeyIterator {
 	return &keyIterator{n, 0}
 }

node.go

@@ -63,6 +63,20 @@ type Node interface {
 	AsString() (string, error)
 	AsBytes() ([]byte, error)
 	AsLink() (Link, error)
+
+	// NodeBuilder returns a NodeBuilder which can be used to build
+	// new nodes of the same implementation type as this one.
+	//
+	// For map and list nodes, the NodeBuilder's append-oriented methods
+	// will work using this node's values as a base.
+	// If this is a typed node, the NodeBuilder will carry the same
+	// typesystem constraints as this Node.
+	//
+	// (This feature is used by the traversal package, especially in
+	// e.g. traversal.Transform, for doing tree updates while keeping the
+	// existing implementation preferences and doing as many operations
+	// in copy-on-write fashions as possible.)
+	NodeBuilder() NodeBuilder
 }
 
 // KeyIterator is an interface for traversing nodes of kind map.