Update TypedServer stage4 demo to the current main

[erszcz]

This will involve using an auxiliary helper together with changes to Gradient.TypedServer.CompileHooks.

Specifically, CompileHooks will have to detect the standard GenServer.handle_call return values instead of just detecting the TypedServer.reply call, as the latter won't be usable at all, since the auxiliary handle cannot accept from at all. This is due to from, due to the use of term() / any() in its definition, disabling exhaustiveness checking. See #96 for more details.