* indent.el: Provide interactive indent-rigidly mode. Use lexical-binding.
(indent-rigidly--current-indentation): New function.
(indent-rigidly-map): New var.
(indent-rigidly): Use it to provide interactive mode.
(indent-region): Add progress reporter.
(tab-stop-list): Make it implicitly extend to infinity by repeating the
last step.
(indent--next-tab-stop): New function to implement this behavior.
(tab-to-tab-stop, move-to-tab-stop): Use it.