Added some more documentation for define_readers and define_writer of the Association and its inherited classes

Fixes #7418.

Method compilation provides better performance and I think the code
comes out cleaner as well.

A knock on effect is that methods that get redefined produce warnings. I
think this is a good thing. I had to deal with a bunch of warnings
coming from our tests, though.

Well, not all of them, but some of them.

I don't think there's much reason for these methods to be private.

We don't need the complexity of to_sentence, and it shouldn't be a bang
method.

It's not really a good idea to have this as a global config option. We
should allow people to specify the behaviour per association.

There will now be two new values:

* :dependent => :restrict_with_exception implements the current
  behaviour of :restrict. :restrict itself is deprecated in favour of
  :restrict_with_exception.
* :dependent => :restrict_with_error implements the new behaviour - it
  adds an error to the owner if there are dependent records present

See #4727 for the original discussion of this.

Move the logic for validation check to the same method, and cache
dependent option in a variable to reuse inside the dependency
configuration methods instead of relying on the options hash.

This fixes active_record_deprecated_finders.

See the CHANGELOG for details.

Fixes #950.

This reverts commit c99d507f.

This change uses Module.redefine_method as defined in ActiveSupport.
Making Module.define_method public would be as clean in the code, and
would also emit warnings when redefining an association. That is pretty
messy given current tests, so I'm leaving it for someone else to decide
what approach is better.

Instead of generating association methods directly in the model
class, they are generated in an anonymous module which
is then included in the model class. There is one such module
for each association. The only subtlety is that the
generated_attributes_methods module (from ActiveModel) must
be forced to be included before association methods are created
so that attribute methods will not shadow association methods.

Use proper objects to do the work to build the associations (adding methods, callbacks etc) rather than calling a whole bunch of methods with rather long names.