Use alloc proposal in parallel mergesort

Also, remove typeclass constraint on `free` since it is unnecessary and enables typechecking

Author: jazullo

src/Array.hs

@@ -99,7 +99,7 @@ makeArray = make
 #endif
 
 {-# INLINE free #-}
-free :: HasPrim a => Array a -. ()
+free :: Array a -. ()
 free = Unsafe.toLinear (\_ -> ())
 
 {-# INLINE flattenCallback #-}

src/DpsMergeSort4Par.hs

@@ -107,8 +107,8 @@ msort' :: (Show a, HasPrimOrd a) =>
   a -> A.Array a -. A.Array a
 msort' anyVal src =
   let !(Ur len, src') = A.size2 src
-      !(src'', _tmp) = msortInplace src' (A.make len anyVal) in
-  case A.free _tmp of !() -> src''
+      !src'' = A.allocScratch len anyVal msortInplace src' in
+  src''
 {-# INLINE msort' #-}
 
 -- finally, the top-level merge sort function