Remove dangerous ST hack for recent base library

GHC 8.0 added a `runRW#` primop to work around the issue that led
to the unsafe reimplementation of `runST` in `unordered-containers`.
Use the standard `runST` for `base` 4.9.0 and above.

This commit *does not* remove the `-fno-full-laziness` invocations
in various modules; doing so will require testing to ensure that it
is good (or at least not bad) for performance.

Author: treeowl

Data/HashMap/Unsafe.hs

@@ -1,4 +1,8 @@
+{-# LANGUAGE CPP #-}
+
+#if !MIN_VERSION_base(4,9,0)
 {-# LANGUAGE MagicHash, Rank2Types, UnboxedTuples #-}
+#endif
 
 -- | This module exports a workaround for this bug:
 --
@@ -12,6 +16,12 @@ module Data.HashMap.Unsafe
     ( runST
     ) where
 
+#if MIN_VERSION_base(4,9,0)
+-- The GHC issue was fixed in GHC 8.0/base 4.9
+import Control.Monad.ST
+
+#else
+
 import GHC.Base (realWorld#)
 import qualified GHC.ST as ST
 
@@ -26,3 +36,4 @@ runSTRep :: (forall s. ST.STRep s a) -> a
 runSTRep st_rep = case st_rep realWorld# of
                         (# _, r #) -> r
 {-# INLINE [0] runSTRep #-}
+#endif