This commit changes bodies of methods generated by `alias_attribute`
along with generating these methods lazily.

Previously the body of the `alias_attribute :new_title, :title` was
`def new_title; title; end`. This commit changes it to
`def new_title; attribute("title"); end`.

This allows for `alias_attribute` to be used to alias attributes named
with a reserved names like `id`:
```ruby
  class Topic < ActiveRecord::Base
    self.primary_key = :title
    alias_attribute :id_value, :id
  end

  Topic.new.id_value # => 1
```