Make Q() return a boolean value based on it's length

[David Cramer]

The goal would be to allow you to do a boolean check on a Q() object to see if it actually contains any statements.

x = Q()

x
<django.db.models.query.Q object at 0x275a410>

bool(x)
True

This would return False, and thus allow you to check the Q() instance to see if it actually has values.

Django falls back just fine with empty Q() objects, this is just a small nicety.

[Malcolm Tredinnick]

Thanks to the magic of time machines, this is already true with Q objects on the queryset-refactor branch. This ticket will be closed when the branch is merged into trunk.

[Malcolm Tredinnick]

(In [7477]) Merged the queryset-refactor branch into trunk.

This is a big internal change, but mostly backwards compatible with existing
code. Also adds a couple of new features.

Fixed #245, #1050, #1656, #1801, #2076, #2091, #2150, #2253, #2306, #2400, #2430, #2482, #2496, #2676, #2737, #2874, #2902, #2939, #3037, #3141, #3288, #3440, #3592, #3739, #4088, #4260, #4289, #4306, #4358, #4464, #4510, #4858, #5012, #5020, #5261, #5295, #5321, #5324, #5325, #5555, #5707, #5796, #5817, #5987, #6018, #6074, #6088, #6154, #6177, #6180, #6203, #6658