Rework mkSortedSerializePlan

> I'm not entirely happy with this interface. I intend to work on this later. 

 _Originally posted by @joaosreis in [#82](https://github.com/tweag/cardano-canonical-ledger/pull/82/files/2ea0874d603df14e06fcf7dd545b94fcca097d52#r2449109754)_