Classes defined in each era should go into Core module

[lehins]

It was discussed that classes are spread around through out the repo and it would be best if each era defined type classes in the Cardano.Ledger.[Era].Core module. Unless of course the type class is used as an internal implementation in some module.

For example newly defined type classes in #2901, eg AlonzoEraTxBody is defined in Cardano.Ledger.Alonzo.TxBody module, but it needs to move into Cardano.Ledger.Alonzo.Core, while instances should still live in Cardano.Ledger.Alonzo.TxBody.

For convenience I also suggest we re-export the whole of Cardno.Ledger.Core module and Cardano.Ledger.[Era].Core from a previous era from it as well. Eg.

module Cardano.Ledger.Shelley.Core
  ( module Cardano.Ledger.Core
  , ShelleyTxBody(..)
  ...
  ) where

module Cardano.Ledger.ShelleyMA.Core
  ( module Cardano.Ledger.Shelley.Core
  , ShelleyMATxBody(..)
  ...
  ) where

[lehins]

CC @TimSheard this was your idea, so just letting you know that I created this ticket