a little bit more lxml performance tweaking

Today I merged the backpointer branch with the lxml trunk, and have been cleaning up a bit more. In particular I've cleaned up some useless extra subclasses that were only necessary to introduce weak reference support to the various classes. I've now removed these subclasses, which cleans things up a bit more.

This also resulted in some performance gains! Not very spectacular ones, but still noticable:

                    nodereg  backptr  cleanups
                    --------------------------

findall('//v')      1.9 s    0.25 s   0.21 s
xpath('//v')        0.76 s   0.21 s   0.19 s
xpath('//v/text()') 0.34 s   0.25 s   0.25 s

Apparent element proxy creation was sped up by a notch. This explains why the //v/text() operation was not affected, as only strings are created as a result of that, not element proxies.

Of course cElementTree is still at 0.13 s for the findall('//v') operation, but as you can see the lxml.etree xpath version is not that far off anymore.