Deconstruct State for handler functions

[msrd0]

Currently, all handler functions take either State or &State as their only parameter, and need to retrieve all information they need in user code. It would be more convenient if we allowed for syntax like this:

route.get("/").to(|(headers, body): (HeaderMap, Body)| { /* create response */ })

This was first suggested by @tanriol in the Gitter chat.