Some core extension methods are not documented

Some core extension methods, like alias_method_chain, are not properly documented. With this I mean that they don't show up properly in the generated API reference HTML and on sites such as rails-doc.org. This is because methods like alias_method_chain are defined directly on the core classes, instead of an ActiveSupport module.

The attached patch fixes this problem. It move some core extension methods into a module under the ActiveSupport::CoreExtensions namespace, instead of extending core classes directly. This is more friendly for API reference generators.