bug#46515: Repeat mode

[Matt Armstrong <matt@rfc20.org>]

First thing: neat idea, and don't listen to me.  :-)

Have you thought about making it more clear to the user that their keys
are now doing different things?  Most successful "modal" interfaces I
have seen have clear indicators.

One idea is to look at the other places in Emacs that already use
`set-transient-map' in this way and try to be "at least as good" as
those. `kmacro' and `indent-rigidly' are two reasonable examples. They
print messages when active that describe the newly active key bindings.
repeat.el doesn't describe the key binding, but it does say a repeat
mode is active. Kmacro is so smart that it aranges for the repeat key to
be based on whatever key the command was invoked with.

As far as this general approach for creating small transient modes, I
can't help but think it is too low level. An approach that had a bit
more scafolding to it would let Emacs' help system describe it, and it
might allow for a consistent way for Emacs to indicade they are active
--- similar to how the conventions under major and minor work for
"heavier" modes.