Use "cookies" from 4.02.1 to make named regexps work in toplevel

[whitequark]

Test code (t.ml):

#require "sedlex";;

let lid = [%sedlex.regexp? "foo"];;

let lex lexbuf =
  match%sedlex lexbuf with
  | lid -> true
  | _ -> false
;;

ocaml <t.ml

[alainfrisch]

Thanks! It would require much more work (esp. because the internal representation currently uses functions), but I'd be more inclined to store the regexp definitions in a more structured way (e.g. by keeping the original Parsetree fragments) rather than through the generic binary marshaling.

[whitequark]

@alainfrisch I thought about it, but I'm certainly not up to an almost complete rewrite of sedlex.

[alainfrisch]

@whitequark You're so pessimistic :-) See commit bceb0d9

[whitequark]

@alainfrisch I tried to do something like that but failed--I guess I did not try hard enough.

You still need the opam changes from this PR, after which it may be closed.