Make `Array` unlifted?!

[sjakobi]

Inspired by https://gitlab.haskell.org/ghc/ghc/-/issues/26525, I wonder if we could simply turn Array into an unlifted type.

newtype Array a = Array { unArray :: SmallArray# a }

Since Array isn't part of our public API, this shouldn't cause any trouble, right?!

(Related: #463)

[treeowl]

It should work for recent enough versions of GHC. Be a bit careful; there were one or two versions early on that didn't unbox those properly at all!