Few more optimisations to cellfun

[Jordi Gutiérrez Hermoso]

I have committed the following change that seems to optimise cellfun a
bit more for the string vs function handle cases:

    http://hg.savannah.gnu.org/hgweb/octave/rev/cf8cd43cdeb3

I noticed, however, that cellfun behaves differently for size and
class when passed as a string vs a function handle. I tried to
preserve this behaviour while optimising all other cases. I didn't see
any regressions with make check, so I went ahead and pushed.

If you notice any brokenness with cellfun in the near future, you can
probably blame me.

- Jordi G. H.