The old names were downright confusing. Some seemed to mean the opposite
of what the class actually did.
The new names follow a consistent nomenclature:
(Forward|Reverse)(ManyToOne|OneToOne|ManyToMany)Descriptor.
I mentioned combinations that do not exist in the docstring in order to
help people who would search for them in the code base.
Field.rel is now deprecated. Rel objects have now also remote_field
attribute. This means that self == self.remote_field.remote_field.
In addition, made the Rel objects a bit more like Field objects. Still,
marked ManyToManyFields as null=True.
