* xpath.c include/libxml/xpath.h: Added an XPath object cache.
  It sits on an xmlXPathContext and need to be explicitely
  activated (or deactivated again) with
  xmlXPathContextSetObjectCache(). The cache consists of 5
  lists for node-set, string, number, boolean and misc XPath
  objects. Internally the xpath.c module will use object-
  deposition and -acquisition functions which will try to reuse
  as many XPath objects as possible, and fallback to normal
  free/create behaviour if no cache is available or if the cache
  is full.
* runsuite.c: Adjusted to deactivate the cache for XML Schema
  tests if a cache-creation is turned on by default for the whole
  library, e.g. for testing purposes of the cache. It is
  deactivated here in order to avoid confusion of the memory leak
  detection in runsuite.c.