Updating *Completions* as you type

[sbaugh]

It would be nice if there was a built-in customization which caused
*Completions* to update as you type, as long as that buffer is visible.
I imagine such a request has been made before, so what is the obstacle
to adding it?

I would like to figure out a solution which everyone is happy with, and
then I would be happy to implement it.  It seems to me that it's just a
matter of, after each keystroke, triggering (for minibuffer completion)
minibuffer-completion-help or (for buffer completion) some new function
to populate *Completions* with all-completions output.  This could (I
guess) be done with after-change-functions, although maybe others have a
better idea.

(Btw, if we had this behavior, it also seems like it would help with
another long-time request: asynchronous completion support.  A
programmed completion table could internally do something asynchronous
and stateful, and accumulate results over time, and return more and more
results each time Emacs calls all-completions/try-completion.  If Emacs
automatically called all-completions with such a programmed completion
table, the resulting behavior would be an acceptable approximation of
asynchronous completion, without having to complicate the programmed
completion API.)