layout: Re-enable parallel layout and refactor boxes significantly

layout: Re-enable parallel layout by removing all `RefCell` instances from `Flow`s; in the process, remove `InlineInfo` in favor of the range-based design that was originally planned and halfway implemented.

Now, the DOM tree structure for inline flows is reflected not by a
series of arrays but instead by a flat list of ranges into the list of
boxes. As part of this, the `border` and `padding` fields, which were
incorrect in the case of inlines and necessitated separate
`noncontent_inline_foo` methods, have been merged into a single
`border_padding` field that is always guaranteed to be correct after
width assignment, even for inlines.

r? @SimonSapin and/or @larsbergstrom

Closes #1280 
Closes #1926
Closes #1999 
Closes #2013
Closes #2018