With support for str representation of date.
Mostly for testing purpose.

for each entity model `Model`, provide a `models_d` strategy that
produces dicts suitable for using as argument for the `Model.from_dict`
factory method, and reimplement the `models` generator using this
former hypothesis generator.

This is needed to help writing low level tests for model entities.

- add a validator for negative_utc (can be True iff offset is 0),
- update the timestamps_with_timezone hypothesis strategy,
- add low-level tests for it.

(pairing with @vlorentz)

Related to T2310

it was still here for bw-compat but should not be necessary any more.

This generates more realistic contents and avoids spurious HashCollisions when
generating a set of objects using these hypothesis strategies, at the cost of
slightly worse "boundary checking" (i.e. we won't check contents with a length >
4096 bytes).

It already is in practice.

The current postgresql model refuses NULL values.

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.

pgsql does not support it.