Make Service::call() take &mut self

[petoknm]

I found it useful in a single threaded environment to have the option to use an FnMut service.

Example:

let mut counter = 0;
Server::bind((address, port))
    .serve_single_thread(|_| {
        counter += 1;
        Response::builder().status(200).body(format!("count = {}", counter))
    })
    .unwrap();

[petoknm]

Just sharing what changes I made and found useful for my somewhat special circumstances. So I will fully understand if you don't want these changes upstream. :)

[reu]

This actually makes a lot of sense! I was so focused on Hyper when I first built this, that I didn't even take into consideration the possibility of having mutable services, which does fit really well into the sync nature of Touche.

I will just give a code review, but I really like the idea, thanks @petoknm!

[reu]

Hey @petoknm I just opened #9 with some small tweaks but this looks great!