when copying text nodes in elements or attributes to the output, if

* libxslt/templates.c libxslt/transform.c libxslt/xslt.c
  libxslt/xsltInternals.h: when copying text nodes in elements
  or attributes to the output, if interning, then reuse the strings
  directly without copying them in the output tree if dicts are
  shared between the context and the output docs. Less allocations
  smaller output document and no copying involved anymore.
Daniel