Commits on Sep 23, 2023

1. consensus: tame TransitionImpossible et al …

   This commit renames TransitionImpossible to TransitionNever for two reasons.
   First, this is a more precise name. Second, this commit refines the
   singleEraTransition method so that TransitionNever is actually an absorbing
   state. Before this commit, TransitionImpossible could arise under certain
   circumstances but then the TransitionInfo for _the same era_ could later become
   TransitionUnknown. But now it is invariant that once the TransitionInfo for an
   era is ever TransitionNever then it always will be and moreover that era will
   indeed never end.
   
   This invariant is enforced in two ways. singleEraTransition can now induce
   TransitionNever, but only when that decision is independent of the ledger
   state, ie it must be a function only of inputs that the HFC does not vary
   (ledger config, EraParams, start of the era). The only other place
   TransitionNever arises is in the Unary.Isomorphism class, and it now has a law
   requiring that singleEraTransition only induce TransitionNever.

   

   nfrisby committed Sep 23, 2023
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