Use NestedFrame for element representation

**Feature request**

Use `NestedFrame` instead of `pd.DataFrame` when accessing a single nested element, e.g. with `nested_series.iloc[0]`. We also need a benchmark for that, because pandas data-frame construction is already a bottle-neck and we wouldn't like to make it much worse.

**Before submitting**
Please check the following:

- [x] I have described the purpose of the suggested change, specifying what I need the enhancement to accomplish, i.e. what problem it solves.
- [ ] I have included any relevant links, screenshots, environment information, and data relevant to implementing the requested feature, as well as pseudocode for how I want to access the new functionality.
- [ ] If I have ideas for how the new feature could be implemented, I have provided explanations and/or pseudocode and/or task lists for the steps.

Locate the nested_series.iloc[0] access path and the existing pd.DataFrame construction and benchmark code. Read how single nested elements are currently represented; the work is done when that access uses NestedFrame and a benchmark measures its construction cost without a major regression.

Written by the indexing model from the issue text.