LH HandlerT Refinement

HandlerT.lh

## HandlerT refinements
```
module spec Yesod.Core.Types where

data HandlerT site m a <p :: User -> Bool> = Yesod.Core.Types.HandlerT {
        unHandlerT :: Yesod.Core.Types.HandlerData site (Yesod.Core.Types.MonadRoute m) -> m a
        }

data variance HandlerT covariant covariant covariant contravariant

measure content :: HandlerT site m a -> a

instance Monad HandlerT where
 >>= :: forall <p :: User -> Bool, f:: a -> b -> Bool>.
        x:HandlerT <p> site m a
     -> (u:a -> HandlerT <p> site m (b <f u>))
     -> HandlerT <p> site m (b<f (content x)>);
 >>  :: x:HandlerT site m a
     -> HandlerT site m b
     -> HandlerT site m b;
 return :: forall <p :: User -> Bool>. a -> HandlerT <p> site m a
 ```
 
 ## liftM implementation
 {-@ liftM :: forall a b <p :: User -> Bool, f:: a -> b -> Bool>.
                   (u:a -> b<f u>)
                -> x: HandlerT <p> App IO a
                -> HandlerT <p> App IO (b<f (content x)>)
@-}
liftM :: (a -> b) -> Handler a -> Handler b
liftM f x =  x >>= \x' -> return (f x')