Make two strict in its key arguments

`two` wasn't strict in its key arguments. We thought this was okay,
because its key arguments are always in WHNF and it's marked
`INLINE`. But `two` is defined as a *recursive* `go` function
(I haven't looked into why), which can't be inlined. I believe
that's the reason GHC doesn't *realize* that the keys are in WHNF.
Anyway, the end result was that `two` would defer the creation of
the `Leaf` values stored in the array, producing very silly thunks.

Fixes haskell-unordered-containers#232

Data/HashMap/Base.hs

@@ -831,10 +831,10 @@ two = go
         | bp1 == bp2 = do
             st <- go (s+bitsPerSubkey) h1 k1 v1 h2 k2 v2
             ary <- A.singletonM st
-            return $! BitmapIndexed bp1 ary
+            return $ BitmapIndexed bp1 ary
         | otherwise  = do
-            mary <- A.new 2 $ Leaf h1 (L k1 v1)
-            A.write mary idx2 $ Leaf h2 (L k2 v2)
+            mary <- A.new 2 $! Leaf h1 (L k1 v1)
+            A.write mary idx2 $! Leaf h2 (L k2 v2)
             ary <- A.unsafeFreeze mary
             return $! BitmapIndexed (bp1 .|. bp2) ary
       where