Calling a method on `Pin<&mut Self>` moves the pin, which means you can't call more methods later. The solution to this is to use `Pin::as_mut`. But it's annoying to have to do that to _every_ call to the `assert_request_eq!` helper macro from `tower-test`, so I made it do it for me.
* Consolidate `limit` layers
- `InFlightLimit` and `RateLimit` are moved into `tower-limit` crate.
- `InFlightLimit` is renamed to `ConcurrencyLimit`.
Fixes#225
