Add AR::Base#valid! method

databases.rake refactoring

Stop using instance exec for type decorators

We are moving this behavior out to an object that we would like to keep
separated from `ActiveRecord::Base`, which means not passing the class
object to it. As such, we need to stop using `instance_exec`, and
instead close over the subclass on global type decorators that are
applied in `Base`.

It allows the code to be more declarative and elegant.

`ActiveRecord::FixtureSet.create_fixtures` can accept an array of
fixture_files.

Before this commit, if `ENV['FIXTURES_PATH']` was set, then `Rails.root`
was used, otherwise the app used `ActiveRecord::Tasks::DatabaseTasks.root`.

Now it is consistent.

Wrap list items to 80 chars and avoid relying on NOTE inside an item
since they are meant to be parsed in paragraphs, not lists.

[ci skip]

Make the apply_inflections method case-sensitive

Basic support for adding and removing foreign keys

The name of the foreign key is not relevant from a users perspective.
Using random names resolves the urge to rename the foreign key when the
respective table or column is renamed.

This allows to create and remove foreign keys without specifying a column.

respect `table_name_prefix` and `table_name_suffix`.