Cache: protocol overestimates the size of some values when inserting in the cache

The protocol overestimates the size of some values, making the test fail. It seems this is in particular the case for BLS values. This has caused flakiness issues (also linked to unwanted non-determinism), see !3580 (merged) !3532 (merged)

An example:

  Dune__exe__Test_script_typed_ir_size.Script_typed_ir_test_error("Computed size for value #202 `(Right 0xab555536e0e4101c76ba79e3af6c4f0063f8fe710f13604ffed9fe40c384ec1d)\n' of type `(or (ticket (or (option int) (or (option unit) timestamp))) bls12_381_fr)\n' is not a good approximation (48 ~/~ 432)")