Creates an iterator for a map. It provides iterative read-only access to the
Removing or adding keys to the underlying map will invalidate all its
iterators. Updating values of existing keys is allowed and the changes
will be immediately reflected in the iterator.
Iterators are designed to be short-lived and not stored, and creating
them is very cheap. Reading data from an iterator is just as fast as
reading directly from the map.
Just like in snapshots the keys are not sorted.
New iterator handle, which must be freed via TrieIterDestroy().