[xslt] scoped result tree fragments

[<thevakoc-xml yahoo com>]

Hello Daniel,

The way I use libxslt results in a large number of RVTs being created (SQL
results returned in XML) and the memory consumption is becoming a large
problem.  As I understand it all RVTs are created and stored in the transform
context (ctxt->tmpRVT) and those persist until the end of the transformation. 
By the end of my longer stylesheets there is a significant amount of "retired"
RVTs held there.

I haven't fully thought this through but was wondering if RVTs could be
stack'ed like variables are, and automatically destructed when they are out of
scope?  I've been poking around the code trying to understand how exactly that
all works.

Any thoughts?