unobtrusive next key prompting

[Le Wang]

I've been using one-key to make my own menus for long key combos I
don't use often.  https://github.com/vapniks/one-key  Maintaining the
menus is a hassle.

John Wiegley recently added https://github.com/kbkbkbkb1/guide-key to
his config and I like it a lot.  Basically, after a timeout, options
for valid next keys are presented.  If something like this came with
Emacs, I think new users will appreciate it very much.

Thoughts?

Best,

-- 
Le