This type offers a serialization function
sexp_of_t that won't force its argument. Instead, it will serialize the
'a if it is available, or just use a custom string indicating it is not forced. Note that this is not a round-trippable type, thus the type does not expose
of_sexp. To be used in debug code, while tracking a Heisenbug, etc.
type nonrec 'a t = 'a t