This was a small self contained piece of the refactoring that I am
working on, which required these objects to be comparable.

This was only used for uniqueness validations. The first usage was in
conjunction with `limit`. Types which cast to string, but are not
considered text cannot have a limit. The second case was only with an
explicit `:case_sensitive => true` option given by the user.

We have several mutable types on Active Record now. (Serialized, JSON,
HStore). We need to be able to detect if these have been modified in
place.

In some cases there is a difference between the two, we should always
be doing one or the other. For convenience, `type_cast` is still a
private method on type, so new types that do not need different behavior
don't need to implement two methods, but it has been moved to private so
it cannot be used accidentally.

- 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

The definition of `write_attribute` in dirty checking ultimately leads
to the columns calling `type_cast` on the value to perform the
comparison. However, this is a potentially expensive computation that we
cache when it occurs in `read_attribute`. The only case that we need the
non-type-cast form is for numeric, so we pass that through as well
(something I'm looking to remove in the future).

This also reduces the number of places that manually access various
stages in an attribute's type casting lifecycle, which will aid in one
of the larger refactorings that I'm working on.

Only `Date` and `Time` are handled.

The types know more about what is going on than the dirty module. Let's
ask them!

Nearly completely implemented in terms of custom properties.
`_before_type_cast` now stores the raw serialized string consistently,
which removes the need to keep track of "state". The following is now
consistently true:

- `model.serialized == model.reload.serialized`
- A model can be dumped and loaded infinitely without changing
- A model can be saved and reloaded infinitely without changing

This removes the case statement in `SchemaDumper` and gives every `Type`
the possibility to control the SchemaDumper default value output.

/cc @sgrif

Many of the methods defined in `AttributeMethods::Serialization` can be
refactored onto this type as well, but this is a reasonable small step.
Removes the `Type` class, and the need for `decorate_columns` to handle
serialized types.

`ActiveRecord::ConnectionAdapters::Type::Value` =>
`ActiveRecord::Type::Value`