Hidden request_info

Here is the PR to make the type request_info abstract. I put that PR because it is not finished yet, maybe we should think of another design and most importantly, it will led to a change in Eliom. It would be better to discuss it.

@Vouillon think we could use an object interface. Since Cohttp only needs specific fields, we could isolate the implementation used by Cohttp and inherit from it in the implemnetation used by Ocsigenserver.