Hemlock undo (bound to C-_) is useless/broken

[gb]

Hemlock had some sort of limited undo functionality; there are still vestiges of it left in some of the buffer-manipulation and command-processing code, though it's likely bitrotted pretty badly. The NSDocument undo mechanisms at least partly work and should be supported; there's no reason to continue to support the old Hemlock undo cruft.

Being able to invoke "undo" via a Hemlock command (ideally bound to C-_) as well as a menu command seems like a reasonable thing to support; having the Hemlock command do anything different than what the menu command does doesn't seem too reasonable, and whatever low-level Hemlock undo code still exists is likely broken and in the way.