ENH: Make set_option into a contextmanager that undos itself on __exit__
Fixes #5618

cc @y-p and @jseabold

I went ahead and simplifed all the prefixing stuff to just change a single `prefix_key` function instead and handled the dynamic docstring by setting the `__doc__` attribute of functions/classes. set_option is now callable _and_ a contextmanager (whee!) so no need to add anything else to the global namespace (if you don't call `__exit__` it doesn't undo itself)

Had to add a metaclass to this because it's the only way to set docstrings on the _class_ level. Doesn't really add that much noise so I think it's fine.
