Prefix fragment types in inline.rs with `Inline`

This is to reduce confusion once we rename `servo::box_::Box` to `servo::fragment::Fragment`.
Convert usages of `Box` in type identifiers to `Fragment`
Rename functions, fields and adjust comments to use `fragment` instead of `box`

I tried to make sure to keep 'box' where it made sense, and alter comments so they still make sense.
Rename `box_.rs` to `fragment.rs`
Improve naming of structs in inline.rs
Improve line index documentation
auto merge of #2497 : bjz/servo/rename-box-type, r=pcwalton

I tried to make sure to keep 'box' where it made sense, and alter comments so they still make sense.

r? @pcwalton