Rename `ActiveRecord::Fixtures` class to `ActiveRecord::FixtureSet`.  Instances of this class normally hold a collection of fixtures (records) loaded either from a single YAML file, or from a file and a folder with the same name.  This change make the class name singular and makes the class easier to distinguish from the modules like `ActiveRecord::TestFixtures`, which operates on multiple fixture sets, or `DelegatingFixtures`, `::Fixtures`, etc., and from the class `ActiveRecord::Fixture`, which corresponds to a single fixture.