Add NamedRoutes combinator

Allows users to directly embed APIs defined as records of routes into
vanilla Servant API types.

E.g.:

```haskell
data MyRoutes mode = MyRoutes
  { version :: mode :- Get '[JSON] Int
  , …
  }

type API = "prefix" :> NamedRoutes MyRoutes :<|> …
```

APIs can thus be recursively defined directly with Generic record types.

servant/src/Servant/API/Generic.hs

@@ -37,6 +37,8 @@ module Servant.API.Generic (
     ToServant,
     toServant,
     fromServant,
+    -- * NamedRoutes combinator
+    NamedRoutes,
     -- * AsApi
     AsApi,
     ToServantApi,
@@ -120,6 +122,9 @@ genericApi
     -> Proxy (ToServantApi routes)
 genericApi _ = Proxy
 
+-- | Combinator for embedding a record of named routes into a Servant API type.
+data NamedRoutes (api :: * -> *)
+
 -------------------------------------------------------------------------------
 -- Class
 -------------------------------------------------------------------------------