canonicalize (xml, xpath, namespace, exclusive, with_comments)
xml is the xml string to be canonicalized
xpath is a string containing an xpath statement to the part of the xml document to be canonicalized, can optionally be null. See the examples provided with libxml2 for further details.
namespace is a string containing a list of namespaces to be included (only used in exclusive canonicalization)
exclusive is an int to specify exclusive canonicalization (1 = exclusive, 0 = non-exclusive)
with_comments is an int specifying whether comments are included in the canonicalized xml (0 = comments not included)
the function returns a string containing the canonicalized xml.