Make `_before_type_cast` actually be before type cast

- The following is now true for all types, all the time
  - `model.attribute_before_type_cast == given_value`
  - `model.attribute == model.save_and_reload.attribute`
  - `model.attribute == model.dup.attribute`
  - `model.attribute == YAML.load(YAML.dump(model)).attribute`
- Removes the remaining types implementing `type_cast_for_write`
- Simplifies the implementation of time zone aware attributes
- Brings tz aware attributes closer to being implemented as an attribute
  decorator
- Adds additional point of control for custom types
Any special reason to inherit from `SimpleDelegator` now?