Instance index key can be an array of value which could contain nil values. To compare two
hash containing nil values (using <=>), it was necessary to dropback to a comparison of strings.

Example: ['a'] <=> [nil] would be converted to a comparison with "['a']" <=> "[nil]"

Method :<=> does not exist on nil values, and using :<=> on arrays will
call :<=> recursively on its children.

Entities stored in @created_instances have all been through assertion. There
is no need to check if they implement retract.

InstanceIndexKey was in global scope.

RoleValues was basically a wrapper for an array, and is now similar to an ordered
hash. The basic functionnality of RoleValues have not been modified, and it should still
be considered as an array on multiple points, but this array will be ordered by its values
automatically.

Refactoring to InstanceIndexKey has been done also to be used by RoleValues. Therefore, its
name has been modified to be ComparableHashKey instead.

Makes hash traversing function such as #each work with a single argument
instead of 2.

Adds information into related_entities. It should contain a RoleValue
object when the role is pointing to many entities (see unique attribute
for roles).

It also adds a way to query entities in a two-way fashion. Which means
you can now search for entities which references the current one, but
also entities which are referenced by the current one.