node/bindnode: subtract all absents in Length at the repr level

For structs at the repr level, we were only subtracting the number of
trailing absents, which isn't right.

Rod's added checks in the codecs caught this as a failure,
thanks to the node/tests suite already having one such case.

node/bindnode/repr.go

@@ -234,6 +234,7 @@ func (w *_nodeRepr) MapIterator() datamodel.MapIterator {
 	switch stg := reprStrategy(w.schemaType).(type) {
 	case schema.StructRepresentation_Map:
 		itr := (*_node)(w).MapIterator().(*_structIterator)
+		// When we reach the last non-absent field, we should stop.
 		itr.reprEnd = int(w.lengthMinusTrailingAbsents())
 		return (*_structIteratorRepr)(itr)
 	case schema.UnionRepresentation_Keyed:
@@ -291,6 +292,17 @@ func (w *_listIteratorRepr) Done() bool {
 	return w.nextIndex >= w.val.Len()
 }
 
+func (w *_nodeRepr) lengthMinusAbsents() int64 {
+	fields := w.schemaType.(*schema.TypeStruct).Fields()
+	n := int64(len(fields))
+	for i, field := range fields {
+		if field.IsOptional() && w.val.Field(i).IsNil() {
+			n--
+		}
+	}
+	return n
+}
+
 func (w *_nodeRepr) lengthMinusTrailingAbsents() int64 {
 	fields := w.schemaType.(*schema.TypeStruct).Fields()
 	for i := len(fields) - 1; i >= 0; i-- {
@@ -307,9 +319,9 @@ func (w *_nodeRepr) Length() int64 {
 	case schema.StructRepresentation_Stringjoin:
 		return -1
 	case schema.StructRepresentation_Map:
-		return w.lengthMinusTrailingAbsents()
+		return w.lengthMinusAbsents()
 	case schema.StructRepresentation_Tuple:
-		return w.lengthMinusTrailingAbsents()
+		return w.lengthMinusAbsents()
 	case schema.UnionRepresentation_Keyed:
 		return (*_node)(w).Length()
 	case schema.UnionRepresentation_Kinded: