Gnus using lexical-binding

[Stefan Monnier]

I'm working on converting all the lisp/gnus files to use
lexical-binding.  I have a first version of the code ready and pushed to
`scratch/lexical-gnus`.  It works for me, but please test it because my
use of Gnus only exercises a small fraction of its functionality
and packages.

And please, after testing it, tell me how it went, regardless if you
encountered problems or not.

I'll be rebasing that branch as I split it into a few separate commits
to clean up its history, so don't expect to be able to track it
with merges.


        Stefan