XPath: Store string length inside string object
XPath: Store string length inside string object

When xpath_string is heap-allocated we always know the length of the
string at some point - it is now stored in the object. This reduces
redundant string length calculations and makes string_value() much faster
in case it has to concatenate strings.

git-svn-id: 3715ede575e2934be0fecc9022d600f6f7ea101b@1053 99668b35-9821-0410-8761-19e4c4f06640

zeux/pugixml,zeux/pugixml,zeux/pugixml,zeux/pugixml