Custom Slides Ignored; Stepper Ignores Steps

react : 6.13.1
spectacle : 6.0.1

I'm trying Spectacle for the first time and ran into this issue [Various Issues w/ Imported Slides #678](https://github.com/FormidableLabs/spectacle/issues/678), which mentions it should be resolved by the `Slide` rewrite. I believe the rewrite has since been released, but the issue is not resolved so I'd like to reopen. Specifically, `Slide`s are discarded if they are not inlined children of `Deck`. The suggested workaround to wrap `Slide` contents in `React.Fragment` allows the `Slide` to render, but causes nested `Stepper`s to ignore their steps and immediately proceed to the next `Slide`.

Please see [this CodeSandbox](https://codesandbox.io/s/custom-slides-ignored-stepper-ignores-steps-871-xbhlb) I've prepared demonstrating the issues I've mentioned.

Start with the linked CodeSandbox reproduction and trace how Slide children are collected by Deck, then follow the nested Stepper behavior when Slide contents are wrapped in React.Fragment. Done means non-inlined Slides render and nested Steppers advance through their steps before moving to the next Slide; add or update coverage if the repository’s existing tests identify a suitable location.

Written by the indexing model from the issue text.