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
Simplify string construction in InterfaceObjectToString.
Use DOMString in a few more places in utils.rs.
Use DOMString instead of ~str in a few places.
Use eq() and eq_slice() less, and map_default() more.
Use push_str instead of operator + for efficiency.
Make some selector matching code a little more consistent.
auto merge of #1703 : Ms2ger/servo/strings-cleanup, r=jdm