When making a map iterator over a directory, calls to .Next() curre…

…ntly return a value of the `FieldHash()` of the dagpb link.

This change has them instead return a `IterLink`.

In both cases, the "exposed" interface of the returned type, `AsLink()` exposes the underlying hash cidLink to the entry.

By using a wrapping `IterLink` type, we provide an opportunity to type-cast the response, and instead of treating the response under it's returned `ipld.Node` interface,
the client has the ability to do something of the form:
```
k, next, err := iterator.Next()
nextSize := next.(*iter.IterLink).Substrate.FieldSize()
```

iter/iter.go

@@ -37,15 +37,15 @@ func (itr *UnixFSDir__MapItr) Next() (k ipld.Node, v ipld.Node, err error) {
 		if itr.transformName != nil {
 			name = itr.transformName(name)
 		}
-		return name, next.FieldHash(), nil
+		return name, &IterLink{next}, nil
 	}
 	nb := dagpb.Type.String.NewBuilder()
 	err = nb.AssignString("")
 	if err != nil {
 		return nil, nil, err
 	}
 	s := nb.Build()
-	return s, next.FieldHash(), nil
+	return s, &IterLink{next}, nil
 }
 
 func (itr *UnixFSDir__MapItr) Done() bool {

iter/iterlink.go

@@ -0,0 +1,67 @@
+package iter
+
+import (
+	dagpb "github.com/ipld/go-codec-dagpb"
+	"github.com/ipld/go-ipld-prime/datamodel"
+	basicnode "github.com/ipld/go-ipld-prime/node/basic"
+)
+
+type IterLink struct {
+	Substrate dagpb.PBLink
+}
+
+func (il *IterLink) AsBool() (bool, error) {
+	return false, datamodel.ErrWrongKind{}
+}
+func (il *IterLink) AsBytes() ([]byte, error) {
+	return nil, datamodel.ErrWrongKind{}
+}
+func (il *IterLink) AsFloat() (float64, error) {
+	return 0.0, datamodel.ErrWrongKind{}
+}
+func (il *IterLink) AsInt() (int64, error) {
+	return 0, datamodel.ErrWrongKind{}
+}
+func (il *IterLink) AsLink() (datamodel.Link, error) {
+	return il.Substrate.FieldHash().AsLink()
+}
+func (il *IterLink) AsString() (string, error) {
+	return "", datamodel.ErrWrongKind{}
+}
+
+func (il *IterLink) IsAbsent() bool {
+	return il.Substrate.IsAbsent()
+}
+func (il *IterLink) IsNull() bool {
+	return il.Substrate.IsNull()
+}
+func (il *IterLink) Kind() datamodel.Kind {
+	return datamodel.Kind_Link
+}
+func (il *IterLink) Length() int64 {
+	return 0
+}
+func (il *IterLink) ListIterator() datamodel.ListIterator {
+	return nil
+}
+func (il *IterLink) LookupByIndex(idx int64) (datamodel.Node, error) {
+	return nil, datamodel.ErrWrongKind{}
+}
+func (il *IterLink) LookupByNode(key datamodel.Node) (datamodel.Node, error) {
+	return nil, datamodel.ErrWrongKind{}
+}
+func (il *IterLink) LookupBySegment(seg datamodel.PathSegment) (datamodel.Node, error) {
+	return nil, datamodel.ErrWrongKind{}
+}
+func (il *IterLink) LookupByString(key string) (datamodel.Node, error) {
+	return nil, datamodel.ErrWrongKind{}
+}
+func (il *IterLink) MapIterator() datamodel.MapIterator {
+	return nil
+}
+func (il *IterLink) Prototype() datamodel.NodePrototype {
+	return basicnode.Prototype__Link{}
+}
+func (il *IterLink) Representation() datamodel.Node {
+	return il
+}