Refactored the internal structures into specialized structures and

* libxslt/attributes.c libxslt/preproc.c libxslt/transform.c
  libxslt/variables.c libxslt/xslt.c libxslt/xsltInternals.h
  libxslt/xsltutils.c libxslt/xsltutils.h:
  Refactored the internal structures into specialized
  structures and adjusted the code to work with those new
  structures. I didn't yet (we should in the future)
  renamed any fields of the old structures in order to avoid
  changing too much code.
  Introduced the internal structure xsltCompilerCtxt to be
  used for storage and control of the compilation.
  Optimized the way lists of in-scope namespaces are created
  and stored; this will now only generate a new list if
  really needed, i.e. if we encounter a ns-decl.
  All this changes here are IFDEFed out with XSLT_REFACTORED.