rlp: return Iterator as non-pointer (#33818)

Most uses of the iterator are like this:

    it, _ := rlp.NewListIterator(data)
    for it.Next() {
        do(it.Value())
    }

This doesn't require the iterator to be a pointer and it's better to
have it stack-allocated. AFAIK the compiler cannot prove it is OK to
stack-allocate when it is returned as a pointer because the methods of
`Iterator` use pointer receiver and also mutate the object.

The iterator type was not exported until very recently, so I think it is
still OK to change this API.

rlp/iterator.go

@@ -25,20 +25,20 @@ type Iterator struct {
 }
 
 // NewListIterator creates an iterator for the (list) represented by data.
-func NewListIterator(data RawValue) (*Iterator, error) {
+func NewListIterator(data RawValue) (Iterator, error) {
 	k, t, c, err := readKind(data)
 	if err != nil {
-		return nil, err
+		return Iterator{}, err
 	}
 	if k != List {
-		return nil, ErrExpectedList
+		return Iterator{}, ErrExpectedList
 	}
-	it := &Iterator{data: data[t : t+c], offset: int(t)}
+	it := newIterator(data[t:t+c], int(t))
 	return it, nil
 }
 
-func newIterator(data []byte) *Iterator {
-	return &Iterator{data: data}
+func newIterator(data []byte, initialOffset int) Iterator {
+	return Iterator{data: data, offset: initialOffset}
 }
 
 // Next forwards the iterator one step.

rlp/raw.go

@@ -122,8 +122,8 @@ func (r *RawList[T]) Empty() bool {
 // ContentIterator returns an iterator over the content of the list.
 // Note the offsets returned by iterator.Offset are relative to the
 // Content bytes of the list.
-func (r *RawList[T]) ContentIterator() *Iterator {
-	return newIterator(r.Content())
+func (r *RawList[T]) ContentIterator() Iterator {
+	return newIterator(r.Content(), 0)
 }
 
 // Append adds an item to the end of the list.