Rewritten to use modern Emacs internals

• Allows rectangle-mark-mode to work as expected
• Uses yank-handler, so no need to replace the yank command
• Improved preservation of point, generally to improve correctness
• Default override for comment-dwim is changed, and now uses prefix to denote the number of lines to comment/uncomment, rather than comment level
• Correctly handles cycling of yanks with yank-pop
• Removed the obsolete whole-line-or-region-mode alias: use either the -local-mode or the -global-mode now.
• Removed the old-fashioned "turn-on" function
• Greatly improved tests

Enormous thanks to @gusbrs for help with this!