Accessor methods for association instances.

Currently the active_record/associations.rb contains code like the following:

ivar = "@#{reflection.name}"

# for reading
association = instance_variable_get(ivar) if instance_variable_defined?(ivar)
if association.respond_to?(:loaded?)
  # ...
end

# and for writing
instance_variable_set(ivar, association)

This patch cleans this up by providing a association_instance_get and a association_instance_set method, which provide the same functionality as described above.

# for reading
if association = association_instance_get(reflection.name)
  # ...
end

# and for writing
association_instance_set(reflection.name, association)

As you can see, it's the same minus the instance_variable_defined? part. Tests have shown that this call is not needed. Please provide feedback if it it needed for a unknown reason.