If a footnote has been output but we then decide to cancel the line it
was output on, we should cancel the output of the footnote itself.
This requires a bit of extra bookkeeping, so we know which footnotes are
relevant, but otherwise largely works using the existing
remove_placeholders code.
Includes a minor refactor of footnote area management in LayoutContext,
and a new testing utility: `tree_position`.
Closes#1564.
