… user/auth (#246)

This creates a `Singleton` pool service which wraps an inner make-service, and queues up all calls while waiting for the one instance to be created, and then hands out clones. This pattern fits HTTP/2 well.

The current implementation mirrors much of how the legacy pool handles connecting with HTTP/2 prior knowledge.

Closes hyperium/hyper#3953

This creates a new `Cache` type that acts as a pool over an inner make-service. Asking the Cache to produce a service will either return a cached one, or create a new one with the inner maker. The returned service is `Cached<S>`, which knows to return the service to the cache upon Drop.

Closes hyperium/hyper#3954

Creates a `Negotiate` type that uses an inner make-service, inspects the result, and then either returns an upgraded service, or fallback, depending on the negotiated. This is somewhat like a `Router`, but instead of routing on the request, it's routing on the response. It's an advanced form of an `Either`. The original use case is that of a connector that can sends ALPN to a destination, and can either comeback with HTTP/2 negotiated, or fallback to HTTP/1.

Closes hyperium/hyper#3955

The `map` pool isn't a typical Service like the other pools, but rather is a stand-alone type that maps requests to keys and connectors. As a kind of router, and it cannot determine which inner service to check for backpressure until the request is made. The map implementation allows customization of extracting a key, and how to construct a connector for that key.

Closes hyperium/hyper#3979