Require that LedgerView family is empty

[nfrisby]

Enforcing this property as in this PR is not actually necessary, but we suspect it may prevent a good deal of confusion.

[nfrisby]

This relatively large diff emerges from merely adding the invariantLedgerViewEmpty :: Proxy p -> LedgerView p -> a method to the ConsensusProtocol class. Everything else is a consequence of that---no other major decisions were made.

I had originally planned to declare data LedgerView p itself as an empty data type. However, our protocol combinators (eg ModChainSel p s) currently rely on LedgerView (ModChainSel p s) ~ LedgerView p, and that would be impossible with an injective LedgerView.

[nfrisby]

I think the major alternative to this approach is to replace all occurrences of Ticked (LedgerView p) with LedgerView p.

Perhaps renaming it to TickedLedgerView p?

[nfrisby]

I should have kept this haddock comment

[nfrisby]

This is the only aspect of the diff that I anticipate trickling down into eg cardano-node.

[amesgen]

Another idea: push the evidence that LedgerView is empty into a superclass constraint instead of into a new member function, eg a simple Generic-based assertion that it has no constructors, or reusing sth like the existing Absurd class (already has Generic deriving).

[nfrisby]

I much prefer this alternative idea #363, so I'm Closing this.