mid: Implement downstream retrograde analysis

Start at a root board with 18 or more stones, the mid solver performs
an exhaustive backwards traversal of all downstream positions, keeping
track of rotations only via super_t.  No standardization is performed,
since 18 stone positions are likely to be quite rigid.

Currently this takes between 15.4 and 20 s for an 18 stone boards.
Wonderfully, this time should be completely invariant to the board.
mid: Cut memory usage in half by taking advantage of parity

Each super_t has 256 bits, but we actually only use half of these
since rmax always flips parity.  Thus, only one parity value is
needed for each slice.

I had expected this to cut computation time as well, but it didn't
have much effect.  This may mean I am weird loop logic limited, not
memory bandwidth.
mid: Minor cleanups, and some discussion