Cookie/Response/Request could use some functional setters

[rgrinberg]

For example updating the expiration of Set_cookie_hdr.t value is not easy. I have similar annoyances with Request/Response and headers. Easiest way to fix this is to add the fieldslib dependency and with fields to the types. Or just add the boilerplate if a new dependency is a no-no.

[avsm]

I've been playing with avsm/ocaml-lens to see if those are better getter/setters instead of fieldslib. But yes, this is a good point. Fieldslib isn't a bad solution either.

[avsm]

It's no longer abstract in the interface in 0.9.12, since there are no invariants protected by the creation function.

[avsm]

I changed my mind and exposed a bunch of fieldslib implementations of request/response and cookie in 0.9.12

[rgrinberg]

Thanks, this is deal. Now updating a cookie within a request is much easier.