add instances of IsString for ghc 7.4.x

tests/KATSalsa.hs

@@ -2,6 +2,7 @@
 module KATSalsa (vectors) where
 
 import qualified Data.ByteString as B
+import Data.ByteString.Char8 ()
 
 key :: B.ByteString
 key = "\xEA\xEB\xEC\xED\xEE\xEF\xF0\xF1\xF2\xF3\xF4\xF5\xF6\xF7\xF8\xF9\xFA\xFB\xFC\xFD\xFE\xFF\x00\x01\x02\x03\x04\x05\x06\x07\x08\x09"

tests/Tests.hs

@@ -5,6 +5,7 @@ import Control.Applicative
 import Data.Byteable
 import Data.ByteString (ByteString)
 import qualified Data.ByteString as B
+import Data.ByteString.Char8 ()
 import Test.Tasty
 import Test.Tasty.QuickCheck
 import Test.Tasty.HUnit