ENH: Use Fletcher to better support strings, structs, and arrays

[tswast]

I recently learned about Fletcher, which provides a pandas extension dtype that is backed by pyarrow.

Since it's backed by arrow, it avoids having to use the object dtype for strings, structs, and arrays. But also, since it's only 0.1, this should probably be opt-in.

Similar to #242, since both these proposals use extension dtypes.

[meredithslota]

The project was archived earlier this year and is no longer supported (https://github.com/xhochy/fletcher/tree/master) — closing.

[tswast]

Indeed. ArrowDtype is now built-in to Pandas. #621 tracks a way to integrate with ArrowDtype in a way consistent with other pandas I/O methods.