Integer is currently a wired-in type for integer-gmp. This requires replicating its inner structure in TysWiredIn.lhs, which makes it much harder to change Integer to a more complex representation (like e.g. for #9281 (closed))

In order to reduce the redundancy (due to wired-in type mirroring) and simplify the code, as suggested by Simon, Integer should no longer be a wired-in type but rather become a known-key-entity.