Next step of refactoring. For more details see bug #341588. I applied the

* libxslt/documents.c libxslt/namespaces.c
  libxslt/preproc.c libxslt/transform.c
  libxslt/xslt.c libxslt/xsltInternals.h libxslt/xsltutils.c:
  Next step of refactoring. For more details see bug #341588.
  I applied the suggestion of Jerome Pesenti to the refactored
  (still IDFEDed out with XSLT_REFACTORED) code: The XPath
  compilation context (accessible via the compilation context)
  is now reused for compilation of expressions at
  compilation-time; this should reduce compilation time to 50%
  for avarage stylesheets.