Add documentation for Line::{bounds, green_zone}
Remove the need to destructure InlineFragments

We do this by moving InlineFragmentMap::fixup into InlineFragments
Remove last methods from InlineFragmentMap
Remove InlineFragmentMap
Use destructuring to clean up InlineFragments::push_all
auto merge of #2553 : bjz/servo/remove-inline-fragment-map, r=pcwalton

This simplifies inline.rs and makes things easier to understand.

I've also added some documentation for `Line`.

r? @pcwalton