Experimental features

[Stefan Monnier]

Release practice during Emacs-21 at least is that new features could only
appear in new major releases, except for a few small exceptions.

I'd like to change that to make released Emacsen evolve faster.
One way to do that would be to introduce the idea of "experimental"
features which could be added to any minor release.

An experimental feature would be disabled by default and the code should be
written in such a way that as long as the feature is enabled, it's clearly
obvious that it cannot have any negative effect.

We'd then provide a way for the user to activate some of the experimental
features from her .emacs file.  After some time, we would promote the new
feature such that it is not experimental any more.


        Stefan