Change Text, Comment and ProcessingInstructions parent name from element to characterdata

All other node's parent type names reflect the actual type of the parent. This change extends that convention to the indicated nodes.

closes #1594
auto merge of #1702 : quandrum/servo/element_to_characterdata, r=jdm

... to characterdata

All other node's parent type names reflect the actual type of the parent. This change extends that convention to the indicated nodes.

closes #1594