Related to P589

Add support to automatically compute identifier in the following object models:
Directory, Release, Revision, Snapshot.

If the identifier is not provided as parameter, it will be computed when the model
is initialized.

we do not really need them to be mutable, plus we gain their instances now
being hashable, so we can add them in set() for example.

Code manipulating a Content object may want to access a hash
of configurable name; this method allows it to do that
without using getattr directly.