Feature: add a newtype for unsafe reference equality `===`

[LiamGoodacre]

Regarding the discussion here: purescript/purescript-prelude@3b0d306

One idea was to define a newtype UnsafeRefEq a = UnsafeRefEq a whose Eq instance uses JavaScript's === comparison operator. If false, then fall back to Eq a.
We can then remove the unsafe === optimization from the Eq a => Eq (Array a) instance.
If anyone wants to recover this behaviour, they can use the newtype.