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