Replace all ~"" with "".to_owned().
auto merge of #2311 : Ms2ger/servo/empty-owned, r=jdm
Replace most ~"string"s with "string".to_owned().
Reorder some comparisons to avoid allocating strings for them.
Call find_equiv to avoid allocation strings.
auto merge of #2312 : Ms2ger/servo/owned-strings, r=SimonSapin
Use str.to_owned() in tests.
~[] to Vec in FontStyle and FontGroup
~[] to Vec in style/media_quieres.rs, selectors.rs, stylesheets.rs
~[] to Vec in main/compositing and associated files
~[] to std::vec::Vec in layout
~[] to Vec in main/layout/construct.rs and associated files
~[] to Vec in main/layout/box_.rs, gfx/text/util.rs, main/layout/text.rs
~[] to Vec in main/layout/layout_task.rs, script/dom/clientrectlist.rs,
script/layout_interface.rs
~[] to Vec in main/layout/wrapper.rs
~[] to Vec in main/compositing/compositor_layer.rs
~[] to Vec in main/constellation.rs
~[] to Vec in main
~[] to Vec in layout flow, block, table, row, rowgroup, wrapper
~[] to Vec in table, table_colgroup, and text