val with_in : ?mode:int ‑> ?flags:Pervasives.open_flag list ‑> string ‑> (char t ‑> 'a) ‑> 'aread filename f opens filename and calls f g,
where g is a clonable generator of characters from the file.
It can be cloned by calling g#save (which saves the position
in the file), and used with g#next. Distinct clones of g shouldn't
be used at the same time (otherwise Failure _ will be raised).
Both the generator and save points are only valid within
the scope in which f is called.