Streamline TypedServer demo `handle/2`

[erszcz]

In #95 we make the demo use an auxiliary handler handle/2 which doesn't take GenServer.from() as a parameter. This is due to the fact that the GenServer.from() is defined using term(), which disables exhaustiveness checking. The Erlang from() does not do that, but it uses an nonempty_improper_list() instead, which full support requires confirmation.

In order to achieve the simplest handler with exhaustiveness checking we would need to:

• Fix Elixir GenServer.from() definition
• Make sure nonempty_improper_list() does not break exhaustiveness checking

[erszcz]

Ahhh, alas, the Erlang reply_tag also uses term() in the last variant:

-opaque reply_tag() :: % As accepted by reply/2
          reference()
        | nonempty_improper_list('alias', reference())
        | nonempty_improper_list(
            nonempty_improper_list('alias', reference()), term()).

This means that we'll have to use an auxiliary function or a case expression to benefit from exhaustiveness checking over messages.