env refactoring: use a custom ADT for missing pers_struct entries
env refactoring: use a custom ADT for missing pers_struct entries

The hope is that a tailor-made algebraic datatype is more readable /
less confusing than using ('a option) directly -- one may confuse
getting None when looking in a table with the Not_found case.

(Suggested by Jérémie Dimino)

gerdstolpmann/ocaml,gerdstolpmann/ocaml,gerdstolpmann/ocaml,gerdstolpmann/ocaml,gerdstolpmann/ocaml