Unified temporal subtraction support

[charettes]

I recently worked on making substraction of temporal fields return a
`DurationField` on all database backends[1]:

class TemporalModel(models.Model):
    from_date = models.DateField()
    to_date = models.DateField()

    from_time = models.TimeField()
    to_time = models.TimeField()

    from_datetime = models.DateTimeField()
    to_datetime = models.DateTimeField()


TemporalModel.objects.annotate(
    date_delta=F('to_date') - F('from_date'),  # Will be a timedelta
    time_delta=F('to_time') - F('from_time'),  # Will be a timedelta
    datetime_delta=F('to_datetime') - F('from_datetime'), # Will be a timedelta
)

Before this change the substraction operator either didn't work (MySQL, SQLite)
or returned inconsistent results (`DateField` - `DateField` returned an `int`
for the number of days on Oracle and PostgreSQL).

Now that all the tests are passing on all backends I'd like to get feedback on
two design decisions I took.

First, should we make substraction of `DateField`s return an integer instead of
a `timedelta` just like Oracle and PostgreSQL do by default? I initialy chose to
return a `timedelta` (use `DurationField` as `output_field`) because while it
made more sense to return an `int` from a SQL point of view I felt like the
former was more appropriate for the level of abstraction the ORM deals with. The
use of `timedelta` also introduces a backward incomptiblity issue for users of
Oracle and PostgreSQL before the unification effort.

My second concerns is about subtraction of temporals of different types. As
noted on the pull request I can see value in implementing the `DateField` and
`DateTimeField` cases but I'm not convinced it's even worth it.

For reference here's the different signatures of the `-` operator of PostgreSQL:

| LHS type  | RHS type  | Return type
| --------- | --------- | -------------
| date      | date      | integer
| date      | time      | timestamp
| date      | timestamp | interval
| time      | date      | _undefined_
| time      | time      | interval
| time      | timestamp | _undefined_
| timestamp | date      | interval
| timestamp | time      | timestamp
| timestamp | timestamp | interval

As usual, thanks for your feedback.

Simon

[1] https://github.com/django/django/pull/5997