Various cleanups

1. Removed unused optional parameter in Fitness_storage
2. Removed useless Lwt usage in Baking
3. Merged Qty_repr in Tez_repr (no need for a functor when it is instantiated only once)
4. Move trace from Script_interpreter to Helpers_services (it is easier to see that it is accessible from RPCs only, it helps reviewing Tickets by ensuring the ~internal flag is set properly at all call sites)
5. Move typecheck_data from Script_ir_translator to Helpers_services (for the same reason)
6. Various comments
7. Extract unpack from the big interpreter function to its own function (was useful in a version of Tickets)
8. Fix FIXMEs in Fixed_point_repr