Support for boxed trait returns

[cetra3]

What I'd like to do is something like this, so I can use some of the struct fields within the response:

extern crate shio;

use shio::prelude::*;
use shio::Handler;

struct TestStruct {}

impl TestStruct {
    pub fn test_closure(&self) -> Box<Handler<Result=Response>> {
        Box::new(|_ctx| Response::with("Hello World"))
    }
}

fn main() {

    let test_struct = TestStruct {};

    Shio::default()
        .route((Method::Get, "/", test_struct.test_closure()))
        .run(":7878").unwrap();
}

This would allow responses to be built up using other struct fields.

But this errors out:

error[E0277]: the trait bound `std::boxed::Box<shio::Handler<Result=shio::Response>>: std::ops::Fn<(shio::Context,)>` is not satisfied
  --> src/main.rs:19:10
   |
19 |         .route((Method::Get, "/", test_struct.test_closure()))
   |          ^^^^^ the trait `std::ops::Fn<(shio::Context,)>` is not implemented for `std::boxed::Box<shio::Handler<Result=shio::Response>>`
   |
   = note: required because of the requirements on the impl of `shio::Handler` for `std::boxed::Box<shio::Handler<Result=shio::Response>>`
   = note: required because of the requirements on the impl of `std::convert::From<(shio::Method, &str, std::boxed::Box<shio::Handler<Result=shio::Response>>)>` for `shio::router::Route`
   = note: required because of the requirements on the impl of `std::convert::Into<shio::router::Route>` for `(shio::Method, &str, std::boxed::Box<shio::Handler<Result=shio::Response>>)`

error: aborting due to previous error

error: Could not compile `shiotest`.

To learn more, run the command again with --verbose.

Using the new impl trait on nightly works OK though:

#![feature(conservative_impl_trait)]
extern crate shio;

use shio::prelude::*;
use shio::Handler;

struct TestStruct {}

impl TestStruct {
    pub fn test_closure(&self) -> impl Handler<Result=Response> {
        |_ctx| Response::with("Hello World")
    }
}

fn main() {

    let test_struct = TestStruct {};

    Shio::default()
        .route((Method::Get, "/", test_struct.test_closure()))
        .run(":7878").unwrap();
}