Add `examples/service_struct_impl.rs`, which provides an up-to-date
example of implementing MakeService and Service on custom types.
The `Svc` struct has a Counter, which demonstrates how to instantiate
shared resources in the `MakeSvc` and pass the resource to the
services. Updates the `examples/README.md` and the doc in
`src/service/mod.rs`.
Closes#1691
