Specify cross-era ticking/forecasting for Cardano

The Consensus Team needs assistance with Issue https://github.com/IntersectMBO/ouroboros-consensus/issues/418.

Right now, there is no separate specification for how to tick/forecast ledger states across era boundaries, there is only code. Other than Byron-to-Shelley, it's "obvious for the most part". But there are plenty of subtleties buried in the implementation.

This Issue is to create a proper specification of the relation that must hold between two ledger states X and Tick(X, S) --- X is the ledger state that results from applying the final block of some era, and S is the slot of the first block of the next era, which will be validated against the Tick(X, S) ledger state.