Relaxed type signature of asks function

It's previous type signature:

  asks :: (e -> a) -> Eff '[Reader e] a

Was very restrictive, therefore, its usage was limited to very few
effect stacks. New type signature:

  asks :: Member (Reader e) effs => (e -> a) -> Eff effs a

Is much more liberal, and allows us to use `asks` in all the places we
would have to use something like `f <$> ask`.

Resolves #7

src/Control/Monad/Freer/Reader.hs

@@ -63,16 +63,16 @@ ask = send Reader
 -- | Request a value of the environment, and apply as selector\/projection
 -- function to it.
 asks
-    :: (e -> a)
+    :: Member (Reader e) effs
+    => (e -> a)
     -- ^ The selector\/projection function to be applied to the environment.
-    -> Eff '[Reader e] a
+    -> Eff effs a
 asks f = f <$> ask
 
 -- | Handler for 'Reader' effects.
 runReader :: Eff (Reader e ': effs) a -> e -> Eff effs a
 runReader m e = handleRelay pure (\Reader k -> k e) m
 
-
 -- | Locally rebind the value in the dynamic environment.
 --
 -- This function is like a relay; it is both an admin for 'Reader' requests,