Add support for c14n serialization

[ndw]

Apparently it'll be an option in Saxon 9.9, so expose it when it's available.

[dwcramer]

This would be great to have. I can set saxon:canonical="yes" on an xsl:output and this mostly works even in Calabash, but I notice that when it comes out of Calabash, the empty tags are collapsed. I've had to move the part of my pipeline where I generate and consume the canonicalized version of some xml outside of the pipeline.