Redo manhattandir to avoid heap allocation and be fast.  This is just the first of several commits that rework the template structure of the event window and the manhattandir stuff.  That hits a lot of stuff and the tree unfortunately will be broken until all the commits go in.
Partial rework of BitField to be O(1) on Read() and Write().  Another tree-is-still-broken commit.
Add second template arg -- R for event window radius -- to ElementTable.  Another tree-is-still-broken commit.
Add second template arg -- R for event window radius -- to EventWindow.  Another tree-is-still-broken commit.
Forgot the manhattandir test stuff.  Another tree-is-still-broken commit.
Up our const-correctness game in Point; also workaround signedness stuff.  Tree still broken (but this commit is safe, I think).
Add some typedefs for common cases in testing, to ease the pain of the extra template args.  Tree still broken.
The rest of the hacks for this round.  Tree should now build again.  (And stuff should run a little faster -- but this is only a downpayment on that.)