Improve cycle-spacing and bind it to M-SPC by default

Improve cycle-spacing and bind it to M-SPC by default

* lisp/bindings.el (esc-map): Bind M-SPC to cycle-spacing instead of
just-one-space.
* lisp/simple.el (delete-space--internal): New function.
(delete-horizontal-space): Use it.
(delete-all-space): New command.
(just-one-space): Implement on its own instead of calling
cycle-spacing with a special flag.
(cycle-spacing--context): Make it a plist instead of a list.  Adapt
docstring accordingly.
(cycle-spacing-actions): New user option.
(cycle-spacing): Rewrite so that it performs the actions in
cycle-spacing-actions instead of the hard-coded ones.
* doc/emacs/killing.texi (characters): Mention and add a variable
index entry for cycle-spacing-actions.
* etc/NEWS: Document that M-SPC is now cycle-spacing instead of
just-one-space.