In some ADT, ordering of constructors is important because a certain
meaning is attached to ordinality (e.g., "clusters with lesser AllocPolicy
are preferred").

To ensure constructor order is not accidentally changed, this commit
introduces assertions that verify the ordering of these types is as
expected.

Signed-off-by: Dato Simó <dato@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>