Update TypedServer demo

[erszcz]

This PR updates the TypedServer demo to use current Gradient (instead of GradualizerEx) and makes sure it's runnable as is without any glitches.

The demo as of this PR still relies on two non-standard features in upstream Gradualizer:

• Check exhaustiveness argument-wise josefs/Gradualizer#391
• Make binary() aka String.t() in Elixir refinable josefs/Gradualizer#392

The aim is to get rid of any customisations and either make the demo conform to the upstream feature set or merge the necessary changes upstream.

[erszcz]

@zuiderkwast This is the example code from the Friday live demo. To make it work the way it does (looking from the utility / UX perspective) josefs/Gradualizer#391 is necessary upstream. I didn't see any breakage in Gradualizer from incorporating that change, but OTOH I'm not yet done with all the relevant literature on the subject, which could highlight potential issues. Just letting you know what my thinking currently is.

[Premwoik]

Looks good! Well done examples :D Thanks for adding the formatter of the nonexhaustive error :)

[Premwoik]

Why comment out this here and in the other places?

[erszcz]

This only makes sense if use GenServer is also enabled at the top, but this in turn leads to a type error reported within GenServer implementation. I don't want this to clutter the output when demoing, but ultimately we would want these uncommented and the error sorted out.

Good point, though, I'll create a ticket to track this.

[erszcz]

Tracked as #64

[zuiderkwast]

@zuiderkwast This is the example code from the Friday live demo. To make it work the way it does (looking the from the utility / UX perspective) josefs/Gradualizer#391 is necessary upstream.

Thanks for inviting me to the presentation! It was very good! About the PR, I'm convinced now. :)