MonadTime: export NominalDiffTime, diffUTCTime and addUTCTime

We export 'DiffTime' and its action on 'Time' through 'addTime'.  We
also export 'UTCTime', but not 'NominalDiffTime' and its action on it.
This patch fixes this discrepency.

io-sim-classes/src/Control/Monad/Class/MonadTime.hs

@@ -2,15 +2,23 @@
 module Control.Monad.Class.MonadTime (
     MonadTime(..)
   , MonadMonotonicTime(..)
+
+  -- * 'DiffTime' and its action on 'Time'
   , Time(..)
   , diffTime
   , addTime
   , DiffTime
+
+  -- * 'NominalTime' and its action on 'UTCTime'
   , UTCTime
+  , diffUTCTime
+  , addUTCTime
+  , NominalDiffTime
   ) where
 
 import           Control.Monad.Reader
-import           Data.Time.Clock (DiffTime, UTCTime)
+import           Data.Time.Clock (DiffTime, NominalDiffTime, UTCTime,
+                   addUTCTime, diffUTCTime)
 import qualified Data.Time.Clock as Time
 import           Data.Word (Word64)
 

io-sim/src/Control/Monad/IOSim/Internal.hs

@@ -49,8 +49,7 @@ import           Data.OrdPSQ (OrdPSQ)
 import qualified Data.OrdPSQ as PSQ
 import           Data.Set (Set)
 import qualified Data.Set as Set
-import           Data.Time (NominalDiffTime, UTCTime (..), addUTCTime,
-                     diffUTCTime, fromGregorian)
+import           Data.Time (UTCTime (..), fromGregorian)
 import           Data.Typeable (Typeable)
 
 import           Control.Applicative (Alternative (..))

io-sim/test/Test/IOSim.hs

@@ -14,7 +14,7 @@ import           Data.Fixed (Fixed (..), Micro)
 import           Data.Graph
 import           Data.Function (on)
 import           Data.List (sortBy)
-import           Data.Time.Clock (DiffTime, picosecondsToDiffTime)
+import           Data.Time.Clock (picosecondsToDiffTime)
 
 import           Control.Exception (ArithException (..))
 import           Control.Monad