detect unconsumed/unsent messages, introduce generated dummy messages

[drahnr]

If a message type is never sent or never received, this leads to From<$ty> implementations being missing.

The implementation here adds a check and throws and error pointint to the messages.

Closes #13

[drahnr]

Note, currently one integration test errors due to exactly that fact:

error: Message MsgStrukt is sent but never received
  --> orchestra/examples/solo.rs:28:40
   |
28 |     #[subsystem(consumes: Plinko, sends: [MsgStrukt])]
   |                                           ^^^^^^^^^

error: Message Plinko is received but never sent
  --> orchestra/examples/solo.rs:28:24
   |
28 |     #[subsystem(consumes: Plinko, sends: [MsgStrukt])]
   |                           ^^^^^^

error: could not compile `orchestra` due to 2 previous errors

[sandreim]

LGTM, but I think we should add a test for this extra check and fix the failing one.

[drahnr]

Just tried with polkadot, and it needs to also address the case where a subsystem only actually sends messages (i.e. BitfieldSigningSubsystem, PvfChecker). Currently this would error out.

I am thinking of allowing an _ for the receiving part, such that the two dummy / non instantiable enums can be deleted

[drahnr]

So the impl details are as follows:

• create an internal dummy message type, if none is specified
• the generated type is not taken into account when doing the graph or sanity checks, and hence the checks will pass
• the channels are still created in a fully connected fashion, just as before, due to some bigger refactoring required to avoid them, which - again - points to Avoid dependency on consumed message type, generate a marker type #11

---

To use this in polkadot, delete the two unused messages paritytech/polkadot#6413

[drahnr]

I'll sign all commits, one sec, I missed that before.