to the class.

decouple the builder classes from the model.  Builder objects should be
easier to reuse now.

`configure_dependency` actually defined callbacks, so rename the method
and move it to the appropriate method.

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.