The patch provides FFI access to Gecko's SeenPtrs type from Rust, in
order to record what has already been measured when measuring Arcs. (The
SeenPtrs must be initialized on the Gecko side because the same table is
reused for measuring all Elements within a window, because Elements can
share ComputedValues.) I have confirmed with DMD that this is working
correctly.
The patch also introduces MallocSizeOfRepeats, which is like
MallocSizeOf but takes a SizeOfState, which holds a SeenPtrs table.
This file has become quite bloated lately. This commit deletes that file in
favor of a set of submodules.
The only noticeable change apart from code move, is converting deep_clone_foo
methods into a trait.
It also unifies logic related to different style rules in the same place.
There's some missing work, specially related to font-face and counter-style, but
I think this is worth landing in the meantime.
