New ‘formatting’ checker

[Ludovic Courtès]

Hi!

‘guix lint’ now has a ‘formatting’ checker, which currently just checks
for tabs, trailing white space, and long lines.  The idea is to remove
the burden of checking those things from reviewers, and to avoid
frustrations caused by reviews that report trailing white space.  :-)

It’s not perfect because it doesn’t know about sexp boundaries, so it
might report about issues in the package that comes next.  It’s OK
though.

Comments welcome!

Ludo’.