[Ecls-list] Some improvements

[Tobias C. Rittweiler]

Gabriel Dos Reis writes:

> If you have an idea about the branch prediction, you can use
> (when compiling with GCC) __builtin_expect.

There were a couple of occasions where I'd have liked there were a CL
declaration for that, too. Perhaps ECL could set a precedent?

Something on the line of

  (if (foop)
      (locally (declare (ext:likely-branch))
        (frob-a))
      (frob-b))

  -T.