Enumeratees - pass terminals variant.

Provides enumeratees that pass terminal markers (EOF) to the inner iteratee.

Most enumeratees, upon receipt of EOF, will enter a done state and return the inner iteratee without sending EOF to it. This allows for composing enumerators as in:

 myEnum extraData i = do
 nested <- enumFile "file" (mapChunks unpacker i)
 inner <- run nested
 enumList extraData inner

if mapChunks unpacker sent EOF to the inner iteratee i, there would be no way to submit extra data to it after runing the result from enumFile.

In certain cases, this is not the desired behavior. Consider:

 consumer :: Iteratee String IO ()
 consumer = liftI (go 0)
   where
     go c (Chunk xs) = liftIO (putStr s) >> liftI (go c)
     go 10 e         = liftIO (putStr "10 loops complete")
                         >> idone () (Chunk "")
     go n  e         = I.seek 0 >> liftI (go (n+1))

The consumer iteratee does not complete until after it has received 10 EOFs. If you attempt to use it in a standard enumeratee, it will never terminate. When the outer enumeratee is terminated, the inner iteratee will remain in a cont state, but in general there is no longer any valid data for the continuation. The enumeratee itself must pass the EOF marker to the inner iteratee and remain in a cont state until the inner iteratee signals its completion.

All enumeratees in this module will pass EOF terminators to the inner iteratees.