[CSL-447] Add ReaderT instance for Fork mock
[CSL-447] Add ReaderT instance for Fork mock

input-output-hk/cardano-sl,input-output-hk/cardano-sl,input-output-hk/pos-haskell-prototype,serokell/tw-rework-sketch,input-output-hk/cardano-sl,input-output-hk/pos-haskell-prototype,input-output-hk/cardano-sl,input-output-hk/pos-haskell-prototype