Simplify bitbase.cpp

Use a std::vector to store positions and
rearrange KPKPosition.

No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>
Speedup KPK bitbase of 25%

Change the way the index is coded so that
now looping from 0 to IndexMax generates
the pawns from RANK_7 down to RANK2.

Becuase positions with pawns at RANK_7
are easily classified as wins/draws, this
small trick allows to reduce the number
of needed iterations from 30 down to 26!

No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>