Helpers for serving swagger.json on server

This is what I've been using:

``` haskell
type WithSwagger api = api :<|> ("swagger.json" :> Get '[JSON] Swagger)

serveWithSwagger :: forall layout. HasServer layout => Swagger -> Proxy layout -> Server layout -> Application
serveWithSwagger spec _ serv = serve (Proxy :: Proxy (WithSwagger layout)) (serv :<|> return spec)
```

This can be probably generalized so that we won't have to depend on `servant-server`:

``` haskell
withSwagger :: forall m api application server. Monad m
=> (Proxy (WithSwagger api) -> (server :<|> m Swagger) -> application) -- serve
-> Swagger -> Proxy api -> server -> application
withSwagger serve spec _ server =
serve (Proxy :: Proxy (WithSwagger layout)) (server :<|> return spec)

-- usage example
withSwagger serve (toSwagger api) api server
```

Start with the issue's serveWithSwagger and proposed withSwagger signatures, then inspect how Proxy api, Swagger, and the server composition operator are exposed. Confirm the generalized helper no longer requires servant-server and validate the usage example with toSwagger api.

Written by the indexing model from the issue text.