Enable safestring for 4.02

Well, having optcomp around may be handy for other things as well, and if you want to avoid it you could also just add `Buffer.to_bytes` to `Utils` and use that instead. If I had known that providing a Bytes module through ocamlfind would encourage people to use unsafe conversions, I probably would have thought twice.

PS: batteries will soon-ish have a release with all those extra functions available in older OCaml versions as well. Would depending on Batteries be a possible choice?