Empty array primitives

Motivation

Currently, there is no way to create an empty array without a value of the element type, since the only primitive to create a new array is newArray#, which requires an element. For lifted element types, this is no problem, since you can use a bottom as a dummy element, but that doesn't work for unlifted types.

Proposal

I propose adding two new primitives:

emptyArray# :: forall {l :: Levity} (a :: TYPE (BoxedRep l)). Array# a
emptySmallArray# :: forall {l :: Levity} (a :: TYPE (BoxedRep l)). SmallArray# a

This should be enough, since you can also use them to create empty mutable arrays (by using thawArray#/thawSmallArray#). One could also go the other way round, adding primitives to create mutable empty arrays instead, but I'd argue that empty immutable arrays are more common (e.g. primitive has emptyArray, but no emptyMutableArray). It might still be beneficial to additionally add primitives for empty mutable arrays though.