Merging scratch/substitute-command-keys to master

[Stefan Kangas <stefan@marxist.se>]

I've been working on translating `substitute-command-keys' from C to
Lisp, and I think it's getting ready to merge to master.  Please see the
branch scratch/substitute-command-keys.

Note that this is a rather close 1 to 1 translation from C to Lisp, so
there should also exist many opportunities to take better advantage of
a more expressive language.

One obvious concern is performance.  Here is what I've used to
benchmark it:

(with-temp-buffer
    (dired-mode)
    (let* ((times 100)
           (result (benchmark-run times
                     (documentation 'dired-mode))))
      (cl-mapcar '/ result (make-list 3 (float times)))))

Old C version:     (0.02171680510 0.2  0.01730526807)
New Lisp version:  (0.02775125134 0.24 0.01882684842)

[It would be useful if someone could help verify the above results.]

I hope the above figures are acceptable.  Hopefully, we will be able
to improve the implementation to be more performant over time.

Finally, note that I didn't yet rip out the old C code, and that the
branch is currently based on the master branch from May.

Thanks in advance for any comments and reviews.

(BTW, do we generally merge branches as-is with their full history or do
we squash them and push as a single commit?)