Use a type-safe protocol both for client-server communication and client-frontend communication

[Mikolaj]

Use the excellent formulation by @DavidEichmann from https://github.com/DavidEichmann/VersionedProtocolExample/blob/master/src/Core.hs. Simplify, for now, by not worrying about protocol versions, compatibility, migration, upgrades. When we go multiplayer over net, we'd need that, too. A good talk about the ideas in the simple setup, without versioning, is at https://skillsmatter.com/skillscasts/14633-45-minute-talk-by-duncan-coutts

Currently, the implementation of our protocol is very rigid, allowing no parallelism between clients and server (though permitting limited parts of clients to run in parallel). This was the fastest (and safest) setup last time I checked, but perhaps for wrong reasons that are no longer relevant for newer GHCs and with the newer parallel GC implementation (9.2 improvements in particular) or can be sidestepped otherwise. I bet there is also some spurious complexity suffered to keep the protocol uniform in the absence of strong enough typing. I remember I had to avoid any (server-validated) multi-step UI interactions and express them instead as many disjoint non-modal steps with partial results saved in global state.

With the formalism, we'd be able to specify the permissible parallelism (e.g., who can do what after sending which message, e.g., if only receiving is permitted or also sending and of what messages exactly) and, within these bounds, experiment with various degrees of parallelism easily and safely and, later on, also extend the protocol.

@JamieFristrom: I have a hunch this may interest you. Let me know and I'd prepare groundwork. No hurry, obviously.