Generalize typing of Array.alloc and HashMap.empty

Replacing response type `Ur b` by simply `b` also checks and enables
uses like
```haskell
let (i, _) = HashMap.empty 100 (\ env -> runState (eval e) env)
```
(see issue tweag#404).

Caveat: I am relying on the linea type checker (which might be in
alpha state, dunno), so there might be reasons unknown to me why this
generalization is invalid.  Test suite passes, though.

src/Data/Array/Mutable/Linear/Internal.hs

@@ -65,8 +65,8 @@ alloc ::
   HasCallStack =>
   Int ->
   a ->
-  (Array a %1 -> Ur b) %1 ->
-  Ur b
+  (Array a %1 -> b) %1 ->
+  b
 alloc s x f
   | s < 0 =
       (error ("Array.alloc: negative size: " ++ show s) :: x %1 -> x)

src/Data/Array/Mutable/Unlifted/Linear.hs

@@ -60,7 +60,7 @@ infixr 0 `lseq` -- same fixity as base.seq
 -- | Allocate a mutable array of given size using a default value.
 --
 -- The size should be non-negative.
-alloc :: Int -> a -> (Array# a %1 -> Ur b) %1 -> Ur b
+alloc :: Int -> a -> (Array# a %1 -> b) %1 -> b
 alloc (GHC.I# s) a f =
   let new = GHC.runRW# Prelude.$ \st ->
         case GHC.newArray# s a st of

src/Data/HashMap/Mutable/Linear/Internal.hs

@@ -135,8 +135,8 @@ empty ::
   forall k v b.
   Keyed k =>
   Int ->
-  (HashMap k v %1 -> Ur b) %1 ->
-  Ur b
+  (HashMap k v %1 -> b) %1 ->
+  b
 empty size scope =
   let cap = max 1 size
    in Array.alloc cap Nothing (\arr -> scope (HashMap 0 cap arr))