Rename TestFixtures' public methods

In ActiveRecord::TestFixtures module, rename the following methods:

`fixture_path`        -> `fixture_directory_path
`fixture_table_names` -> `fixture_set_names
(`fixture_table_names` can be used in some future version for something else, for example to return a hash of the form `{ "fixture_set/name" => :fixture_table_name, ... }`
`fixture_class_names` -> `fixture_model_names`
`set_fixture_class`   -> `set_fixture_model`

`fixture_path`, `fixture_table_names`, `fixture_class_names` are deprecated.

`set_fixture_class` is aliased without deprecation.