Expose all cookie directives

[actionshrimp]

• Expose all the HTTP cookie directives, not just expiration
• Remove the defaults from set_cookies and use Set_cookie_hdr.make's defaults. This is the same for the expiration (both are Session), but path is no longer fixed/defaulted to /.
• Use Set_cookie_hdr.t as the internal representation of the cookie used by the middleware as it has all the fields we want and a sexp representation.

NB: encoding of the cookie data is now done when adding to the Env rather than at the point of serialisation - this might actually make debugging a bit easier as it will be clearer that opium is encoding itself rather than something lower level? But happy to change this back if that seems sensible.

[actionshrimp]

I've put the defaulting of path back to "/" to avoid breaking existing apps, as otherwise it apparently defaults to the request URI's path rather than 'nothing' (i.e. = to "/"), I misunderstood here!

[rgrinberg]

Thanks.