hyper-util: move connect under legacy module?

[seanmonstar]

I suspect an eventual next-gen Client would be something like this:

pub struct Client<M> {
    // Hi there. So, let's take a 0.14.x hyper::Client, and build up its layers
    // here. We don't need to fully expose the layers to start with, but that
    // is the end goal.
    //
    // Client = MakeSvcAsService<
    //   SetHost<
    //     Http1RequestTarget<
    //       DelayedRelease<
    //         ConnectingPool<C, P>
    //       >
    //     >
    //   >
    // >
    make_svc: M,
}

Which would likely mean some improvements and changes to the connect module. In order to break less people, we could move what's currently in connect to be at hyper_util::client::legacy::connect.

[kaffarell]

Opened up a PR!