Extend session API

[aronerben]

Various functions in web_session.ml can only be used for either Response.t or Request.t, for example val find : ... -> Request.t -> string option. For symmetry purposes, extend the API so all these functions have duals, one for requests and one for responses (where it makes sense).

Additionally, rethink the semantic for set_value, as it currently also takes a request. Should the session be maintained this way? An alternative is to just delete the request argument and leave that handling to the user. Find out what's more ergonomic.