Destination array newtype wrapper

[Divesh-Otwani]

In Data.Array.Destination, we should replace

newtype DArray a = DArray (MVector RealWorld a)

with

data DArray a where
  DArray :: !MVector RealWorld a -> DArray a 

[aspiwack]

And remove most (all?) calls to Unsafe.toLinear.

[aspiwack]

The original thinking is this comment #287 (comment)

But to widen the horizon a bit, we should probably never write a newtype when creating a linear abstraction over an unrestricted API. At least not if it forces us to use Unsafe.toLinear. Using Unsafe.toLinear when a newtype is involved is pretty dangerous. And likely to be incorrect.

We usually really need a data with an unrestricted field to mediate between the linear and unrestricted world.

I originally wrote this newtype. And I have not the slightest memory of why. I don't know if I thought about it or if I just wrote it out of habit from non-linear Haskell. But either way, I don't think, today, that this newtype is restricted.