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?