Introduce jsid_to_str to simplify jsval_to_str callers that have an id.
Simplify InterfaceObjectToString.
Rewrite jsval_to_str and introduce jsval_to_domstring.
Add some basic debug info for dumping flows.
auto merge of #1212 : larsbergstrom/servo/dump_flows, r=brson

With @ryanhc

Will add more debugging information for RenderBoxes in InlineFlows and `dump_str` for each of the RenderBoxes later.
auto merge of #1206 : Ms2ger/servo/strings, r=jdm

I'll probably do a followup to actually pass non-nullable strings to the native functions.
fix #1211: for table-related elements, handle temporarily as 'block' display intead of 'inline' display