tower::Service and async methods about tower

[withoutboats]

@carllerche has pointed out the issue of bounds: sometimes you want to be able to say that S::Future: Send or S::Future: 'static and so on. Whatever minimal impl trait / existential type feature we implemented would need to support this somehow.

[Nemo157]

Here's a quick test of using the current existential type support for one service implementation: master...Nemo157:async-await-existential

It compiles, but hits an ICE while compiling tests. In other experiments with existential types that I've done I've hit issues around internal lifetimes that blocked compiling at all, luckily using async seems to sidestep those issues .

[carllerche]

I don't believe there currently are any actions to take from this issue, so I'm going to close it. Please let me know if this is an incorrect conclusion.