Make ComputedStyle fields private and add getters.

This isolates layout code from upcoming refactoring in properties.rs.mako.
Un-disable and fix some warnings in the style crate.
Remove CowArc and use the new Arc::make_unique() method instead.
Move some of the box splitting logic into inline.rs
Use a SplitInfo struct instead of a tuple to make the code more self-documenting
Move box splitting by new-line to inline.rs
Improve function naming
Update to owned::Box
Use constructor for SplitInfo to reduce code repetition
Update TODOs
Formatting fixes
auto merge of #2494 : bjz/servo/split-to-width, r=pcwalton

This will make removing the box splitting logic easier to implement. I am also using a `SplitInfo` struct for returning the split results, which should make the code more self-documenting. In the future the `TextRun` should not be returned, and `SplitInfo` should only require a single `CharIndex`, but one thing at a time...

@pcwalton r?
Submodule updates for #2137
Update submodules for #2492
auto merge of #2495 : mbrubeck/servo/2137-submodule-updates, r=larsbergstrom
Derive Clone for ChildFrameTree instead of implementing it manually.
auto merge of #2496 : zwarich/servo/deriving-clone, r=jdm
Request header and postdata support for XHR
auto merge of #2493 : Manishearth/servo/xhr-post, r=jdm

Blocks #2282
Remove unnecessary pubs from script_task