This PR just a proposal, hence being a draft, but I think its something
we should at least consider for tower-service 1.0.

The idea is to remove the blanket `Service` impls for `&mut S` and
`Box<S>` and replace them with an impl for `F where FnMut(Request) ->
Future<Output = Result<Response, Error>>`, and then remove `service_fn`.

I personally think and impl for async fns is more valuable mainly to
hammer home the "services are just async fns" line.

I haven't yet encountered any actual need for the previous impls.
`Ready` was the only thing in tower that used it and it could be easily
changed to not require them.

If we decide to do this I think we should consider doing the same for
`Layer`, i.e. remove `impl Layer for &'a L` and make `Fn(S) -> S2` impl
`Layer`.

This came out of some talk in #650.