Add undo

[krobelus]

Add the readline function undo which is bound to \c_ (control + / on
some terminals). Redoing the most recent chain of undos is supported,
redo is bound to \e/ for now.

Closes #1367.
This approach should not have the issues discussed in #5897.

Every single modification to the commandline can be undone individually,
except for adjacent single-character inserts, which are coalesced,
so they can be reverted with a single undo. Coalescing is not done for
space characters, so each word can be undone separately.

When moving between history search entries, only the current history
search entry is reachable via the undo history. This allows to go back
to the original search string with a single undo, or by pressing the
escape key.
Similarly, when moving between pager entries, only the most recent
selection in the pager can be undone.

TODO: (I'm not sure about the first three)

• decide on the redo hotkey; \e/ does not seem to work on xterm
  (it is tempting to add emacs-style redo but that would probably be less popular)
• vi mode: find some suitable bindings (u and \cr are taken)
• vi mode: find out how to bind in insert mode (which should then also work in the pager's search field)
• update documentation
• User-visible changes noted in CHANGELOG.md

These areas should be mostly alright based on what I tried but there may be room for improvement:

• make sure the undo grouping behavior works nicely in practise
• make sure the interaction with history search feels right
• make sure the interaction with the pager feels right
• add some tests (interactive tests are missing - expect-based tests are not very nice to write last time I tried)

I think this also allows us to get rid of cycle_command_line and cycle_cursor_pos because we can obtain those from the history.

[ridiculousfish]

This is awesome!! The code is solid 💯.

[krobelus]

Thanks!
I also made the terminal flash when there is nothing to undo/redo (flash on an empty commandline seems to do nothing here so that's not noticeable for undo).
Merging to master; the current behavior feels reasonable for a start.
vi-mode keybindings and a more portable redo hotkey should be added eventually