Implement `tf.data.Iterator` methods and properties in `tf.contrib.eager.Iterator`.

Also fixes `tf.contrib.eager.Iterator` so that it can work with
datasets containing `tf.SparseTensor` components.

This change allows you to use the `get_next()` method, `output_types`
property, `output_shapes` property, and `output_classes` property from
`tf.data.Iterator` when constructing a `tf.contrib.eager.Iterator`;
and therefore makes it easier to write code that operates on an
`Iterator` in both eager and graph mode.

PiperOrigin-RevId: 181350797