Configuration helpers (wad: Adding use-package to core)

[Stefan Monnier <monnier@iro.umontreal.ca>]

BTW,

While I'm happy to see `use-package` just reached GNU ELPA and I'm sure
even more people would be happy if it made it into Emacs itself,
`use-package` is solving an old problem and I hope someone will improve
it (or some replacement) to solve new problems.

Some of the problems I think need solving in users's config files are:

- flymake: currently, flymake tends to flag a lot of configuration code
  because that code will be riddled with `setq` on variables that don't
  exist (yet) and calls to functions that don't exist (yet) either.
  A configuration system like use-package/setup/leaf provides some of
  the info needed to resolve this problem, but those packages have yet
  to be tweaked to explain to flymake what's going on.

- idempotent: a good configuration system should make it "hard" to
  write non-idempotent code, and should aim to make it so re-reading
  your init file will give you almost the same result as
  restarting Emacs.
  E.g. if you change a hook function, the result should not be to have
  both the old hook function and the new hook function on that hook.
  Or if you remove the "add-hook" code, re-reading the init file should
  end up removing the hook that was previously added.


        Stefan