Make RangeIndex trait more general
Improve Range method documentation and field and parameter names
Use a tuple of fragment and glyph indices as the index type for LineBoxes

Glyph indices are currently not tracked, once they are, they will allow us to implement faster line breaks in the future.
Add some debug assertions
auto merge of #2443 : bjz/servo/linebox_range, r=pcwalton

Glyph indices are currently not tracked, once they are, they will allow us to implement faster line breaks in the future.

cc. @pcwalton