Remove a 'unsafeCoerce' from 'Unsafe.coerce' (#330)

Previously, the linear `Unsafe.coerce` was implemented using two
`unsafeCoerce`'s, one to get a function `a -> b`, and another one to
cast that function to be a linear one.

After the unsafe equality proofs change[1], we have a
`unsafeEqualityProof` function to cast two types to be the same without
looking at the value, so we can linearly return it, removing the need
for the outer `unsafeCoerce`.

This removes one allocation from the generated STG code for
'Data.Array.Unlifted.get', improving the performance slightly.

[1]: https://gitlab.haskell.org/ghc/ghc/-/commit/74ad75e87317196c600dfabc61aee1b87d95c214

src/Unsafe/Linear.hs

@@ -1,5 +1,8 @@
 {-# LANGUAGE KindSignatures #-}
 {-# LANGUAGE RankNTypes #-}
+{-# LANGUAGE GADTs #-}
+{-# LANGUAGE TypeApplications #-}
+{-# LANGUAGE ScopedTypeVariables #-}
 {-# LANGUAGE TypeInType #-}
 {-# LANGUAGE LinearTypes #-}
 
@@ -29,8 +32,11 @@ import qualified Unsafe.Coerce as NonLinear
 import GHC.Exts (TYPE, RuntimeRep)
 
 -- | Linearly typed @unsafeCoerce@
-coerce :: a %1-> b
-coerce = NonLinear.unsafeCoerce NonLinear.unsafeCoerce
+coerce :: forall a b. a %1-> b
+coerce a =
+  case NonLinear.unsafeEqualityProof @a @b of
+    NonLinear.UnsafeRefl -> a
+{-# INLINE coerce #-}
 
 -- | Converts an unrestricted function into a linear function
 toLinear