Draft: use slots smaller than word as tag for smaller unboxed sums
Saves some space, especially when storing a bunch of unpacked enums.
To clarify: This is beneficial for unboxed sums being used to {-# UNPACK #-}
things into heap objects. This MR doesn't change the calling convention when passing/returning unboxed tuples/sums with smaller-than-word fields.
Edited by Luite Stegeman