Provide Functor & Applicative instances for Partial

This follows the advice from

 http://www.haskell.org/haskellwiki/Functor-Applicative-Monad_Proposal#Missing_superclasses

and silences the single compile warning currently occuring with GHC HEAD.

cabal-install/Distribution/Client/Tar.hs

@@ -71,7 +71,8 @@ import Data.Int      (Int64)
 import Data.Bits     (Bits, shiftL, testBit)
 import Data.List     (foldl')
 import Numeric       (readOct, showOct)
-import Control.Monad (MonadPlus(mplus), when)
+import Control.Applicative (Applicative(..))
+import Control.Monad (MonadPlus(mplus), when, ap, liftM)
 import qualified Data.Map as Map
 import qualified Data.ByteString.Lazy as BS
 import qualified Data.ByteString.Lazy.Char8 as BS.Char8
@@ -676,6 +677,13 @@ partial :: Partial a -> Either String a
 partial (Error msg) = Left msg
 partial (Ok x)      = Right x
 
+instance Functor Partial where
+    fmap          = liftM
+
+instance Applicative Partial where
+    pure          = return
+    (<*>)         = ap
+
 instance Monad Partial where
     return        = Ok
     Error m >>= _ = Error m