* actix_web::fs is now found in the actix-files crate.

* NotFoundHandler is now implemented using ErrorHandlers and the
path to 404.html is provided to the not_found() function using
actix_web::App::data. This setup allows for additional StatusCodes
to be handled in the future, if desired.

* Cleanly serving index.html files for requests to directories now
appears to be possible simply by using index_file("index.html") in
the static files service, so handle_directory() is removed.