This will make it less painful to add additional properties, which
should persist across writes, such as `name`.
Conflicts:
activerecord/lib/active_record/attribute_set.rb
Move behavior of `read_attribute` to `AttributeSet`
Conflicts:
activerecord/lib/active_record/attribute_set.rb
activerecord/test/cases/attribute_set_test.rb
Moved `Builder` to its own file, as it started looking very weird once I
added private methods to the `AttributeSet` class and the `Builder`
class started to grow.
Would like to refactor `fetch_value` to change to
```ruby
self[name].value(&block)
```
But that requires the attributes to know about their name, which they
currently do not.
