Identity trait represents data which can be used
as identifiers. Given two identifiers we just need
to determine if they are equal (represent the same entity)
or not.
The IntoIdentityIterator trait allows to create a sequence of
identifiers starting with a given identifier value and using a
given function which computes a new identifier.
The PartialOrdIdentity trait represents data which can be
used for concurrent actions. Given two such identifiers, we
can tell if they are equal and we could potentially compare them.