Replace the singly-linked z-axis with a doubly-linked list,
and reimplement all z-axis moves as O(1) functions.
Eliminate ncplane_move_{above/below}_unsafe(), as there are no
longer unsafe moves.
* ncplane_at_* and ncplane_at_cursor_*
We had notcurses_at_yx() expanding into three distinct parts of
the cell structure, and ncplane_at_yx() / ncplane_at_cursor()
writing directly to a cell. It was annoying to remember which
was which. The latter two now have a signature matching
notcurses_at_yx(), while the old functionality has been moved
to ncplane_at_yx_cell() and ncplane_at_cursor_yx(). #476
* introduce doctest over googletest #202
* call dtester in in targets
* doctest conversion #202
* channel.cpp -> doctest #202
* egcpool tests -> doctest #202
* input tests to doctester
* zaxis -> doctest
* drone: always define LANG
* libav to doctest #202
* panelreel tests to doctest #202
* spec that a C++17 compiler is now required for doctest #202
* enmetric tests -> doctest #202
* fade tests -> doctest #202
* notcurses test case -> doctest #202
* last conversion to doctest #202
* finish move to doctest #202
* drone: set up make test
* planereels tester #180
* suppress_banner in all tests
* tabletcb: start passing back tablet
* properly initialize fbbytes stat
* panelreel: logic fixes#178
* install all testing data
