tree walker evaluator

[John W. Eaton]

On  6-Jan-2009, John W. Eaton wrote:

|   * I haven't tried to do any timings, so I don't know how this change
|     affects performance compared to the current eval methods in the
|     tree classes.

Here are some timings.  I ran make check on my system with the old
evaluator:

  77.56user 18.73system 1:37.80elapsed 98%CPU (0avgtext+0avgdata 0maxresident)k
  29074inputs+5928outputs (112major+5161147minor)pagefaults 0swaps

and with the new evaluator (using the patch I posted yesterday, no
other changes):

  75.61user 18.78system 1:37.86elapsed 96%CPU (0avgtext+0avgdata 0maxresident)k
  63288inputs+5896outputs (345major+5137036minor)pagefaults 0swaps

So if anything, there is a minor improvement, and perhaps we can even
do better than this with a few more changes.

Does anyone object to me applying this set of changes?

jwe